Battle for Iowa

1. Battle for Iowa

2. Workers take down a U.S. flag following a campaign stop by Hillary Clinton at the Family Fun Center in Adel, Iowa, January 27, 2016. REUTERS/Brian Snyder

3. Marco Rubio speaks at the Sheraton Hotel in West Des Moines, Iowa, January 26, 2016. REUTERS/Aaron P. Bernstein

4. A box of flyers encouraging caucusing for Donald Trump at a campaign rally at the University of Iowa in Iowa City, January 26, 2016. REUTERS/Scott Morgan

5. Hillary Clinton speaks at a campaign event in Cedar Falls, Iowa, January 26, 2016. REUTERS/Rick Wilking

6. Mike Huckabee speaks during a Huckabee Huddle campaign event at the Marshalltown Senior Citizens Center in Marshalltown, Iowa, January 27, 2016. Huckabee was the winner of the Iowa Republican Presidential Caucus in 2008. REUTERS/Carlos Barria

7. "Marvelous Marquee Mike" performs a parody of the Van Halen song "Jump" outside a campaign rally with Donald Trump in Marshalltown, Iowa, January 26, 2016. REUTERS/Brian Snyder

8. Jeb Bush looks at a message on his Apple Watch after speaking at an Iowa Caucus Presidential Candidate Briefing at Nationwide Insurance in Des Moines, January 27, 2016. REUTERS/Scott Morgan

9. A supporter holds a sign as 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

10. A campaign sign for Ted Cruz is seen on the side of the road in Osceola, Iowa, January 26, 2016. REUTERS/Jim Young

11. Hillary Clinton takes the stage to speak during the Scott County Democratic Party's Red, White and Blue Dinner at the Mississippi Valley Fairgrounds in Davenport, Iowa, January 23, 2016. REUTERS/Scott Morgan

12. A picture of Donald Trump hangs outside a house in West Des Moines, Iowa, January 15, 2016. REUTERS/Jim Young

13. Senator Marco Rubio casts a shadow while speaking at a campaign stop in Des Moines, Iowa, January 25, 2016. REUTERS/Brian Snyder

14. The reflection of the campaign bus for Bernie Sanders is reflected in a window as he speaks at a town hall in Iowa Falls, Iowa, January 25, 2016. REUTERS/Mark Kauzlarich

15. Hillary Clinton takes a photo with a supporter at a campaign event in Decorah, Iowa, January 26, 2016. REUTERS/Rick Wilking

16. Donald Trump is joined onstage by Maricopa County Sheriff Joe Arpaio at a campaign rally in Marshalltown, Iowa, January 26, 2016. REUTERS/Brian Snyder

17. A supporter of Bernie Sanders holds a sign as she listens at a town hall at the University of Northern Iowa in Cedar Falls, Iowa, January 24, 2016. REUTERS/Mark Kauzlarich

18. Ted Cruz and his family acknowledge the crowd at Faith Baptist Bible College in Ankeny, Iowa, January 23, 2016. REUTERS/Aaron P. Bernstein

19. Carly Fiorina speaks at a campaign stop at Sip Coffee and Wine Bistro in Mount Pleasant, Iowa, January 26, 2016. REUTERS/Scott Morgan

20. People listen to Hillary Clinton speaking at a campaign event in Oskaloosa, Iowa, January 25, 2016. REUTERS/Rick Wilking

21. Mike Huckabee visits a gun shop during a campaign event in Hiawatha, Iowa, January 25, 2016. REUTERS/Jim Young

22. Supporters of Bernie Sanders listen as Sanders speaks at a town hall at the University of Northern Iowa in Cedar Falls, Iowa, January 24, 2016. REUTERS/Mark Kauzlarich

23. Ted Cruz and former Governor of Texas Rick Perry listen to remarks at a campaign event in Osceola, Iowa, January 26, 2016. REUTERS/Jim Young

24. Actress Lena Dunham campaigns for Hillary Clinton at Eight Seven Central screen printers in Des Moines, Iowa, January 9, 2016. REUTERS/Brian C. Frank

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

26. Donald Trump speaks a campaign event in Pella, Iowa, January 23, 2016. REUTERS/Jim Young

27. Protestors interrupt a campaign rally with Donald Trump in Marshalltown, Iowa, January 26, 2016. REUTERS/Brian Snyder

28. Hillary Clinton holds up a glass of water as she struggles to contain a coughing fit as she speaks at the Jewish Federation of Greater Des Moines while campaigning in Des Moines, Iowa, January 25, 2016. REUTERS/Rick Wilking

29. A volunteer at a Bernie Sanders' campaign event wears a shirt decorated with Sanders' image in Fort Dodge, Iowa, January 19, 2016. REUTERS/Jim Young

30. Attendees listen to Ted Cruz speak at a campaign event in Centerville, Iowa, January 26, 2016. REUTERS/Jim Young

31. Marco Rubio waits backstage while being introduced at a campaign stop in Des Moines, Iowa, January 25, 2016. REUTERS/Brian Snyder

32. Audience members listen as Donald Trump speaks at a campaign rally in Marshalltown, Iowa, January 26, 2016. REUTERS/Brian Snyder

33. Marco Rubio speaks at an American Legion Hall in Oskaloosa, Iowa, January 26, 2016. REUTERS/Aaron P. Bernstein

34. Staff for 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

35. Wendy Keen of Iowa City listens as Hillary Clinton speaks during the Women for Hillary Get Out the Caucus event at Buford Garner Elementary School in North Liberty, Iowa, January 24, 2016. REUTERS/Scott Morgan

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

37. Ted Cruz listens to his introduction at a campaign event in Centerville, Iowa, January 26, 2016. REUTERS/Jim Young

38. Bernie Sanders attends a campaign event at the United Steelworkers Local in Des Moines, Iowa, January 26, 2016. REUTERS/Carlos Barria

39. Ted Cruz arrives at a campaign event in Independence, Iowa, January 25, 2016. REUTERS/Jim Young

40. Ted Cruz speaks at a campaign event in Osceola, Iowa, January 26, 2016. REUTERS/Jim Young

41. A woman wipes tears from her eyes after asking Bernie Sanders a question at a town hall in Iowa Falls, Iowa, January 25, 2016. REUTERS/Mark Kauzlarich