President Obama leads Mitt Romney 47 percent to 45 percent among all voters in a new NPR poll. But the two are tied among voters from 12 battleground states. Carolyn Kaster/Evan Vucci/AP hide caption

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The U.S. Supreme Court on the eve of a hearing about the Florida presidential election recount, Nov. 30, 2000. The justices later ruled 5-4 in the case of Bush v. Gore, effectively deciding the outcome of the presidential race. Alex Wong/Getty Images hide caption

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Wisconsin Republican Gov. Scott Walker (right) celebrates his win over Democratic challenger Tom Barrett at Tuesday night's victory party in Waukesha, Wis. Morry Gash/AP hide caption

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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

itoggle caption Marvin Gentry /Reuters /Landov

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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