Marco Rubio Marco Rubio

Democratic presidential candidate Hillary Clinton speaks at a rally in Marshalltown, Iowa. Clinton is narrowly ahead of Sanders, but within the margin of error in the final Iowa poll. Andrew Harnik/AP hide caption

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Andrew Harnik/AP

Hillary Clinton at a campaign event in Davenport, Iowa. Clinton has seen the race in Iowa tighten but is hoping for a boost from the endorsement of the state's largest newspaper. Brendan Hoffman/Getty Images hide caption

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Brendan Hoffman/Getty Images

Students from Manchester University visit the caucus exhibit at Iowa's state museum in Des Moines Scott Detrow/NPR hide caption

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Scott Detrow/NPR

Politics 101: A College Class Gets Up Close And Personal With The Iowa Caucuses

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Republican presidential candidate Sen. Marco Rubio, R-Fla. holds a rifle during a campaign stop Jan. 15, 2016, at Sturm, Ruger & Co., in Newport, N.H. Matt Rourke/AP hide caption

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Matt Rourke/AP

Establishment Republicans Scramble To Top Trump

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Donald Trump (left) and Ted Cruz joke about Trump's comments that Cruz was a "maniac" and didn't have the right temperament to be president. They played nice then, but will they again? John Locher/AP hide caption

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John Locher/AP

William Gordon, left, helps Steve Wrona as he looks at guns while visiting the K&W Gunworks store in Delray Beach, Fla., on Tuesday, the day President Obama announced executive action on guns. Joe Raedle/Getty Images hide caption

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Joe Raedle/Getty Images

An American soldier carrying equipment walks past an American flag at Incirlik Air Base in Adana, Turkey, as U.S. Air Force war planes are used against ISIS. Anadolu Agency/Getty Images hide caption

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Anadolu Agency/Getty Images

Ted Cruz visited NPR headquarters on Tuesday to talk with Steve Inskeep about his plans for protecting the U.S. from terrorist attacks and his views on climate change. Ariel Zambelich/NPR hide caption

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Ariel Zambelich/NPR

Cruz Won't Criticize Trump But Offers His Own Plan To Bar Refugees

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Marco Rubio stands alone during a Republican presidential debate in Milwaukee on Tuesday. He spoke to NPR about a hot-button issue in that debate — immigration. Jeffrey Phelps/AP hide caption

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Jeffrey Phelps/AP