Republican presidential candidate Mitt Romney walks past Donald Trump's airplane as he arrives in Las Vegas on Tuesday, where he met with Trump for a fundraiser. Mary Altaffer/AP hide caption

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Protesters march outside the Abraham Lincoln Presidential Hotel where Wisconsin Gov. Scott Walker is speaking to the Illinois Chamber of Commerce on April 17 in Springfield, Ill. Walker faces Democrat Tom Barrett in a recall election June 5. The events in the state over the next four weeks could be a sign of where the U.S. is headed in the months ahea Seth Perlman/AP hide caption

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Republican presidential candidate, former U.S. Sen. Rick Santorum speaks at a campaign rally at Four Seasons Sheraton in Mars, Pa., Monday night. Rival Mitt Romney won the Washington, D.C., Maryland and Wisconsin primaries. Jeff Swensen/Getty Images hide caption

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Amy Brighton from Medina, Ohio, who opposes the new health care law, rallies in front of the Supreme Court on Tuesday. Charles Dharapak/AP hide caption

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After his losses in Alabama and Mississippi on Tuesday, Newt Gingrich will face increasing pressure to drop out of the GOP race. Here he waves to supporters after speaking at a rally in Hoover, Ala., on Tuesday. Marvin Gentry /Reuters /Landov hide caption

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Former Massachusetts Gov. Mitt Romney addressed supporters at a Super Tuesday rally in Boston on Tuesday. His home state was one of the six he won Tuesday night. Jessica Rinaldi /Reuters /Landov hide caption

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Former Massachusetts Gov. Mitt Romney waves to the crowd as he is introduced at the start of Wednesday night's Republican presidential debate in Mesa, Ariz. Ross D. Franklin/AP hide caption

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Republican presidential hopeful and former U.S. Sen. Rick Santorum speaks to supporters, flanked by his daughter, Elizabeth (left), and wife, Karen. Whitney Curtis/Getty Images hide caption

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Jan White, left, Brenda Robertson, center, and Janet Freixas, right, count paper ballots at the headquarters of the Douglas County Republican Party Saturday in Minden, Nev., following county-wide Nevada caucus meetings. Ted S. Warren/AP hide caption

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Mitt Romney at his state campaign headquarters Tuesday in Tampa, Fla. Joe Raedle/Getty Images hide caption

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On Thursday night in Jacksonville, Fla., Mitt Romney (right) went after Newt Gingrich from the start on topics such as immigration and colonizing the moon. Matt Rourke/AP hide caption

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