Former Massachusetts Gov. Mitt Romney (left) and Texas Gov. Rick Perry got into a heated exchange about immigration during Tuesday's GOP presidential debate in Las Vegas.

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Business executive Herman Cain, left, and former Massachusetts Gov. Mitt Romney during a debate Tuesday (Oct. 11, 2011) in New Hampshire.

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Herman Cain shakes hands with a family at a Petersburg, Fla. book signing, Wednesday, Oct. 5, 2011.

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Republican presidential candidate Herman Cain during a Sept. 7 debate at the Ronald Reagan Presidential Library. Kevork Djansezian/Getty Images hide caption

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Former Pennsylvania Sen. Rick Santorum during Thursday night's Republican presidential debate in Orlando. Mark Wilson/Getty Images hide caption

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