The campaign bus for U.S. Democratic presidential candidate Bernie Sanders is reflected in a window as he speaks at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

The campaign bus for U.S. Democratic presidential candidate Bernie Sanders is reflected in a window as he speaks at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders listens to a question at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders hugs a supporter at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders listens to his introduction off stage as he waits to speak at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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Staff for U.S. Democratic presidential candidate Bernie Sanders listen in a doorway as he speaks at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (L) speaks while supporters (R) listen at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (L) hugs David Karayof, a student at Iowa State University, after speaking as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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Supporters of U.S. Democratic presidential candidate Bernie Sanders photograph his bus as it backs out of a street outside Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders greets supporters after speaking during a campaign event at Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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Kenny Jackson takes a smoke break ahead of U.S. Democratic presidential candidate Bernie Sanders' campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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Supporters from the Nurses for Bernie Super PAC wave signs as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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Gus Feldman cheers during U.S. Democratic presidential candidate Bernie Sanders' speech at a campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the media after a campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives for lunch at Drake Diner with members of his staff ahead of his final bus tour ahead of the Iowa caucuses in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders looks over the lunch menu at Drake Diner in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Paula Pink, a dog owned by Sue Spicer, wears a t-shirt supporting U.S. Democratic presidential candidate Bernie Sanders at his Iowa campaign headquarters in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Volunteers and staff for U.S. Democratic presidential candidate Bernie Sanders work at his Iowa campaign headquarters in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Simon Bracey-Lane from London, United Kingdom works at the front desk of the Iowa campaign headquarters of U.S. Democratic presidential candidate Bernie Sanders in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Actor Justin Long (L) does a Snapchat recording with MaryAlice Parks of ABC at the Iowa campaign headquarters for U.S. Democratic presidential candidate Bernie Sanders in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to staff and volunteers at his Iowa campaign headquarters in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Billy Hunter, a volunteer for U.S. Democratic presidential candidate Bernie Sanders, holds a door open while waiting for Sanders to arrive at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives with staff at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (C) hugs his wife Jane Sanders (L) while actress Susan Sarandon surveys the overflow room at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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A supporter of U.S. Democratic presidential candidate Bernie Sanders asks campaign staff if she can give Sanders a cash donation at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Supporters of U.S. Democratic presidential candidate Bernie Sanders listen as he speaks at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Staff with U.S. Democratic presidential candidate Bernie Sanders' campaign hold a curtain open as he speaks at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Brokk Canton, 3, (C) hands out American flags to his family at the encouragement of his mother Anna Canton (L) ahead of U.S. Democratic presidential candidate Bernie Sanders' campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Actress Susan Sarandon introduces U.S. Democratic presidential candidate Bernie Sanders (not shown) at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (3rd L) and his wife Jane Sanders take the stage at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders is seen through a piece of architecture as he speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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A supporter of U.S. Democratic presidential candidate Bernie Sanders holds a sign at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders waves to the crowd after speaking at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Supporters watch as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders sighs as he finishes his speech at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders leaves a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Lorraine Williams (R), owner of Cafe Dodici, hugs U.S. Democratic presidential candidate Bernie Sanders at a campaign event at the cafe in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders leaves after speaking at a campaign event in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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A staffer for U.S. Democratic presidential candidate Bernie Sanders prepares for his arrival at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders takes a selfie with a woman outside at a campaign event at a field office in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders walks to the stage at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (R) speaks as activist Cornel West (L) looks on at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West goes to hand U.S. Democratic presidential candidate Bernie Sanders a bottle of water as he speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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A crowd stands and cheers as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Vicki Hall (L), her son Rylon, 11, and husband Randy Hall listen as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West claps as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West (C) cheers as U.S. Democratic presidential candidate Bernie Sanders (R) speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (R) takes the stage with his wife Jane (L) after being introduced by activist Cornel West (C) at a campaign rally in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally cheer as Sanders (not shown) speaks in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally wave American flags as Sanders speaks in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders signs a piece of artwork after speaking at a campaign rally in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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People dismantle the signage on stage after U.S. Democratic presidential candidate Bernie Sanders' campaign rally in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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A member of U.S. Democratic presidential candidate Bernie Sanders' staff pours hot water on spots of ice ahead of his arrival at a campaign rally in Manchester, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives at a campaign rally in Manchester, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West takes the stage to introduce U.S. Democratic presidential candidate Bernie Sanders at a campaign rally in Manchester, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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A member of U.S. Democratic presidential candidate Bernie Sanders' staff waits for his arrival at a campaign event in a residence's garage in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in a residence's garage in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders leaves a garage after speaking at a campaign event in a residence in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders laughs as he speaks to his wife Jane (not shown) on the campaigns' bus after speaking at a campaign event in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (R) speaks with his wife Jane on the campaigns' bus after a campaign event in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives at a campaign event at Wartburg College in Waverly, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Sabrina Meecham, 5, takes a photo of U.S. Democratic presidential candidate Bernie Sanders as he speaks at a campaign event at Wartburg College in Waverly, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West introduces U.S. Democratic presidential candidate Bernie Sanders (not shown) at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Karen Monahan listens as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Anna Koch claps as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders kisses his wife Jane after speaking at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally and concert await the arrival of Sanders at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally and concert sing along with the band Vampire Weekend while awaiting the arrival of Sanders at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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A supporter of U.S. Democratic presidential candidate Bernie Sanders holds a sign before the start of a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Cadence Bartsch (C), 3, watches the concert on stage ahead of U.S. Democratic presidential candidate Bernie Sanders speech at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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The crowd cheers as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (C) sings "This Land is Your Land" on stage with the band Vampire Weekend to close a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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American flags and a cup of water sit on the podium after U.S. Democratic presidential candidate Bernie Sanders speech at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders greets supporters as he heads to the stage at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Alli Scholl, a supporter of U.S. Democratic presidential candidate Bernie Sanders, listens as he speaks at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Rev. Lisa Dietrich (C) and Isabella Dietrich listen as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Tucker Dunn (C), 7, listens as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders greets supporters after speaking at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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A flag waves in the wind outside a house down the street from where U.S. Democratic presidential candidate Bernie Sanders was schedule to speak in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders walks down the street surrounded by family, staff, and media before a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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A television reporter records outside U.S. Democratic presidential candidate Bernie Sanders' bus before a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Larry Kunc, a supporter of U.S. Democratic presidential candidate Bernie Sanders, watches for the candidate to arrive at a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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A supporter listens as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Supporters photograph U.S. Democratic presidential candidate Bernie Sanders (not shown) as he speaks at a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (R) listens to the response from the audience during an interview with Matt Lauer at a campaign rally at Iowa State University in Ames, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Family of U.S. Democratic presidential candidate Bernie Sanders (from L to R), step-daughters Heather Driscoll, Heather Titus, Carina Driscoll, wife Jane Sanders listen as he speaks at a campaign rally at Iowa State University in Ames, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Jon Hathaway shows off a hat full of signatures from U.S. presidential candidates after a Bernie Sanders campaign rally at Iowa State University in Ames, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives at his final campaign rally the night before the Iowa Caucuses in Des Moines, Iowa on Jan. 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Kelsey Johnson (C), a supporter of U.S. Democratic presidential candidate Bernie Sanders, cheers as he speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Myles Carlos, a supporter of U.S. Democratic presidential candidate Bernie Sanders, cheers as Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders waves to the crowd after speaking at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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U.S. Republican presidential candidate Mike Huckabee (2nd L) prays before lunch with supporters at Drake Diner in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders waves to the crowd as he hugs his wife Jane after speaking at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders greets supporters after at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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Supporters of Democratic U.S. presidential candidate Bernie Sanders wait for him to arrive at the final caucus night rally in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

 Photos of a sign for a caucus location outside  Newton, Iowa days after the Iowa Caucuses on February 4, 2016.

Photos of a sign for a caucus location outside Newton, Iowa days after the Iowa Caucuses on February 4, 2016.

 The campaign bus for U.S. Democratic presidential candidate Bernie Sanders is reflected in a window as he speaks at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich
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 Photos of a sign for a caucus location outside  Newton, Iowa days after the Iowa Caucuses on February 4, 2016.

The campaign bus for U.S. Democratic presidential candidate Bernie Sanders is reflected in a window as he speaks at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders listens to a question at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders hugs a supporter at a town hall in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders listens to his introduction off stage as he waits to speak at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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Staff for U.S. Democratic presidential candidate Bernie Sanders listen in a doorway as he speaks at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (L) speaks while supporters (R) listen at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at Iowa State University as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (L) hugs David Karayof, a student at Iowa State University, after speaking as a part of the Iowa State University Presidential Caucus Series in Iowa Falls, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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Supporters of U.S. Democratic presidential candidate Bernie Sanders photograph his bus as it backs out of a street outside Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders greets supporters after speaking during a campaign event at Grinnell College in Grinnell, Iowa January 25, 2016. REUTERS/Mark Kauzlarich

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Kenny Jackson takes a smoke break ahead of U.S. Democratic presidential candidate Bernie Sanders' campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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Supporters from the Nurses for Bernie Super PAC wave signs as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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Gus Feldman cheers during U.S. Democratic presidential candidate Bernie Sanders' speech at a campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to the media after a campaign event at United Steelworkers Local 310L in Des Moines, Iowa January 26, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives for lunch at Drake Diner with members of his staff ahead of his final bus tour ahead of the Iowa caucuses in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders looks over the lunch menu at Drake Diner in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Paula Pink, a dog owned by Sue Spicer, wears a t-shirt supporting U.S. Democratic presidential candidate Bernie Sanders at his Iowa campaign headquarters in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Volunteers and staff for U.S. Democratic presidential candidate Bernie Sanders work at his Iowa campaign headquarters in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Simon Bracey-Lane from London, United Kingdom works at the front desk of the Iowa campaign headquarters of U.S. Democratic presidential candidate Bernie Sanders in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Actor Justin Long (L) does a Snapchat recording with MaryAlice Parks of ABC at the Iowa campaign headquarters for U.S. Democratic presidential candidate Bernie Sanders in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks to staff and volunteers at his Iowa campaign headquarters in Des Moines, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Billy Hunter, a volunteer for U.S. Democratic presidential candidate Bernie Sanders, holds a door open while waiting for Sanders to arrive at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives with staff at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (C) hugs his wife Jane Sanders (L) while actress Susan Sarandon surveys the overflow room at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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A supporter of U.S. Democratic presidential candidate Bernie Sanders asks campaign staff if she can give Sanders a cash donation at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Supporters of U.S. Democratic presidential candidate Bernie Sanders listen as he speaks at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Staff with U.S. Democratic presidential candidate Bernie Sanders' campaign hold a curtain open as he speaks at a campaign rally in Fairfield, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Brokk Canton, 3, (C) hands out American flags to his family at the encouragement of his mother Anna Canton (L) ahead of U.S. Democratic presidential candidate Bernie Sanders' campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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Actress Susan Sarandon introduces U.S. Democratic presidential candidate Bernie Sanders (not shown) at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (3rd L) and his wife Jane Sanders take the stage at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders is seen through a piece of architecture as he speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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A supporter of U.S. Democratic presidential candidate Bernie Sanders holds a sign at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders waves to the crowd after speaking at a campaign rally in Burlington, Iowa January 28, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Supporters watch as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders sighs as he finishes his speech at a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders leaves a campaign rally at Iowa Wesleyan University in Mount Pleasant, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Lorraine Williams (R), owner of Cafe Dodici, hugs U.S. Democratic presidential candidate Bernie Sanders at a campaign event at the cafe in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders leaves after speaking at a campaign event in Washington, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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A staffer for U.S. Democratic presidential candidate Bernie Sanders prepares for his arrival at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders takes a selfie with a woman outside at a campaign event at a field office in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Muscatine, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders walks to the stage at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (R) speaks as activist Cornel West (L) looks on at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West goes to hand U.S. Democratic presidential candidate Bernie Sanders a bottle of water as he speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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A crowd stands and cheers as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Vicki Hall (L), her son Rylon, 11, and husband Randy Hall listen as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West claps as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West (C) cheers as U.S. Democratic presidential candidate Bernie Sanders (R) speaks at a campaign event in Davenport, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (R) takes the stage with his wife Jane (L) after being introduced by activist Cornel West (C) at a campaign rally in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally cheer as Sanders (not shown) speaks in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally wave American flags as Sanders speaks in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders signs a piece of artwork after speaking at a campaign rally in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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People dismantle the signage on stage after U.S. Democratic presidential candidate Bernie Sanders' campaign rally in Dubuque, Iowa January 29, 2016. REUTERS/Mark Kauzlarich

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A member of U.S. Democratic presidential candidate Bernie Sanders' staff pours hot water on spots of ice ahead of his arrival at a campaign rally in Manchester, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives at a campaign rally in Manchester, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West takes the stage to introduce U.S. Democratic presidential candidate Bernie Sanders at a campaign rally in Manchester, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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A member of U.S. Democratic presidential candidate Bernie Sanders' staff waits for his arrival at a campaign event in a residence's garage in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in a residence's garage in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders leaves a garage after speaking at a campaign event in a residence in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders laughs as he speaks to his wife Jane (not shown) on the campaigns' bus after speaking at a campaign event in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (R) speaks with his wife Jane on the campaigns' bus after a campaign event in Charles City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders arrives at a campaign event at Wartburg College in Waverly, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Sabrina Meecham, 5, takes a photo of U.S. Democratic presidential candidate Bernie Sanders as he speaks at a campaign event at Wartburg College in Waverly, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Activist Cornel West introduces U.S. Democratic presidential candidate Bernie Sanders (not shown) at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Karen Monahan listens as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Anna Koch claps as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders kisses his wife Jane after speaking at a campaign rally in Cedar Rapids, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally and concert await the arrival of Sanders at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Attendees of U.S. Democratic presidential candidate Bernie Sanders' campaign rally and concert sing along with the band Vampire Weekend while awaiting the arrival of Sanders at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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A supporter of U.S. Democratic presidential candidate Bernie Sanders holds a sign before the start of a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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Cadence Bartsch (C), 3, watches the concert on stage ahead of U.S. Democratic presidential candidate Bernie Sanders speech at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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The crowd cheers as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders (C) sings "This Land is Your Land" on stage with the band Vampire Weekend to close a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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American flags and a cup of water sit on the podium after U.S. Democratic presidential candidate Bernie Sanders speech at a campaign rally and concert at the University of Iowa in Iowa City, Iowa January 30, 2016. REUTERS/Mark Kauzlarich

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U.S. Democratic presidential candidate Bernie Sanders greets supporters as he heads to the stage at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Alli Scholl, a supporter of U.S. Democratic presidential candidate Bernie Sanders, listens as he speaks at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Rev. Lisa Dietrich (C) and Isabella Dietrich listen as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

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Tucker Dunn (C), 7, listens as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders greets supporters after speaking at a campaign rally in Waterloo, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

A flag waves in the wind outside a house down the street from where U.S. Democratic presidential candidate Bernie Sanders was schedule to speak in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders walks down the street surrounded by family, staff, and media before a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

A television reporter records outside U.S. Democratic presidential candidate Bernie Sanders' bus before a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Larry Kunc, a supporter of U.S. Democratic presidential candidate Bernie Sanders, watches for the candidate to arrive at a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

A supporter listens as U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Supporters photograph U.S. Democratic presidential candidate Bernie Sanders (not shown) as he speaks at a campaign event in Marshalltown, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders (R) listens to the response from the audience during an interview with Matt Lauer at a campaign rally at Iowa State University in Ames, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Family of U.S. Democratic presidential candidate Bernie Sanders (from L to R), step-daughters Heather Driscoll, Heather Titus, Carina Driscoll, wife Jane Sanders listen as he speaks at a campaign rally at Iowa State University in Ames, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Jon Hathaway shows off a hat full of signatures from U.S. presidential candidates after a Bernie Sanders campaign rally at Iowa State University in Ames, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders arrives at his final campaign rally the night before the Iowa Caucuses in Des Moines, Iowa on Jan. 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Kelsey Johnson (C), a supporter of U.S. Democratic presidential candidate Bernie Sanders, cheers as he speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Myles Carlos, a supporter of U.S. Democratic presidential candidate Bernie Sanders, cheers as Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders waves to the crowd after speaking at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/HUCKABEE

U.S. Republican presidential candidate Mike Huckabee (2nd L) prays before lunch with supporters at Drake Diner in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders waves to the crowd as he hugs his wife Jane after speaking at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders greets supporters after at a campaign rally in Des Moines, Iowa January 31, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to volunteers, supporters, and staff at his campaign's Iowa headquarters in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Democratic caucus-goers partake in the 2016 Iowa caucus at Stilwell Junior High School in West Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

Supporters of Democratic U.S. presidential candidate Bernie Sanders wait for him to arrive at the final caucus night rally in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

USA-ELECTION/SANDERS

U.S. Democratic presidential candidate Bernie Sanders speaks to the crowd at a rally on caucus night in Des Moines, Iowa February 1, 2016. REUTERS/Mark Kauzlarich

Photos of a sign for a caucus location outside Newton, Iowa days after the Iowa Caucuses on February 4, 2016.

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