Republican presidential candidates, from left, Bobby Jindal, Rick Santorum, George Pataki, and Lindsey Graham take the stage during the CNBC Republican presidential debate at the University of Colorado on Wednesday. Mark J. Terrill/AP hide caption

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Preparations underway in the Media Center at the third Republican debate on the campus of the University of Colorado in Boulder, Colo. Robyn Beck/AFP/Getty Images hide caption

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People can now contribute to presidential campaigns with just a few taps on a smartphone. Jonathan Alcorn/AFP/Getty Images hide caption

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Ohio Gov. John Kasich's supporters have spent more on TV ads than any other presidential candidate, despite a late entry into the race. New Day For America/YouTube hide caption

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Donald Trump speaks to the media at the U.S.-Mexico border in July. The Libre Initiative's letter doesn't call out any candidates by name, but Trump has called for mass deportation and an end to birthright citizenship for immigrants. LM Otero/AP hide caption

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Republican presidential candidates from left, Jim Gilmore, Lindsey Graham, Bobby Jindal, Rick Perry, Rick Santorum, Carly Fiorina, and George Pataki take the stage for a pre-debate forum at the Quicken Loans Arena, Thursday, Aug. 6, 2015, in Cleveland. Seven of the candidates have not qualified for the primetime debate. Andrew Harnik/AP hide caption

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A supporter waves a Cuban flag in front of the country's embassy after it reopened for the first time in 54 years on July 20 in Washington, D.C. The embassy was closed in 1961 when U.S. President Dwight Eisenhower severed diplomatic ties with the island nation after Fidel Castro took power in a communist revolution. Mark Wilson/Getty Images hide caption

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Wisconsin Gov. Scott Walker offers cheese and ice cream to guests in his hospitality room at the Iowa Lincoln Day Dinner in Des Moines on Saturday. Charlie Neibergall/AP hide caption

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