Mitt Romney

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FILE - In this April 23, 2014, file photo, Republican senatorial candidate Thom Tillis responds during a televised debate at WRAL television studios in Raleigh, N.C. The struggle for control of the Republican Party is getting an early voter test in North Carolina, where former presidential nominee Mitt Romney and tea party favorite Rand Paul on Monday, May 5 pushed their own candidates for the right to challenge Democratic Sen. Kay Hagan in November.(AP Photo/Gerry Broome, Pool)

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** FILE ** Republican presidential candidate and former Massachusetts Gov. Mitt Romney arrives to his election night rally, Wednesday, Nov. 7, 2012, in Boston. President Obama defeated Romney. (AP Photo/Elise Amendola)

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Former Republican presidential candidate Mitt Romney, second from left, holds on news conference to endorse Idaho Gov. Butch Otter, right, Sen. Jim Risch, second from right, and Rep. Mike Simpson, left, after a fundraising lunch at the Powerhouse Event Center on Thursday, March 20, 2014, in Boise, Idaho. (AP Photo/The Idaho Statesman, Joe Jaszewski) LOCAL TV OUT (KTVB 7)

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Former Republican presidential candidate Mitt Romney, second from left, holds on news conference to endorse Idaho Gov. Butch Otter, right, Sen. Jim Risch, second from right, and Rep. Mike Simpson, left, after a fundraising lunch at the Powerhouse Event Center on Thursday, March 20, 2014, in Boise, Idaho. (AP Photo/The Idaho Statesman, Joe Jaszewski) LOCAL TV OUT (KTVB 7)

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Ann and Mitt Romney arrive at the third annual NFL Honors at Radio City Music Hall on Saturday, Feb. 1, 2014, in New York. (Photo by Evan Agostini/Invision for NFL/AP Images)