Vice President Biden and his Republican opponent, Rep. Paul Ryan of Wisconsin, participate in the vice presidential debate at Centre College in Danville, Ky., Thursday. Charlie Neibergall/AP hide caption

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Sen. Lloyd Bentsen (D-Texas) shakes hands with Sen. Dan Quayle (R-Ind.) before their vice presidential debate at the Omaha Civic Auditorium in Omaha, Neb., on Oct. 5, 1988. Ron Edmonds/AP hide caption

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Weeds were already growing in the parking lot outside the GM assembly plant in Janesville, Wis., on May 4, 2009. Scott Olson/Getty Images hide caption

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A delegate shows his support for Paul Ryan on a night dominated by the acceptance speech of the vice presidential nominee. Becky Lettenberger/NPR hide caption

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Republican vice presidential nominee Paul Ryan and his wife, Janna (at right), along with two of their children and his mother, Betty, on stage Wednesday in Tampa. Brendan Smialowski /AFP/Getty Images hide caption

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Republican vice presidential candidate, Rep. Paul Ryan waves with his family Wednesday at the GOP convention in Tampa. Also on stage: his daughter Liza, sons Charlie and Sam Ryan, wife Janna and mother Elizabeth. Win McNamee/Getty Images hide caption

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Republican presidential candidate Mitt Romney speaks at a town hall meeting in Grand Junction, Colo., on July 10. Romney said he has "nothing hidden" in his taxes. Nicholas Kamm/AFP/Getty Images hide caption

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House Budget Committee Chairman Paul Ryan, R-Wis., shakes hands with Federal Reserve Chairman Ben Bernanke at the close of the committee's hearing on the state of the economy in February 2011. J. Scott Applewhite/AP hide caption

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Republican presidential candidate Mitt Romney (right) and running mate Rep. Paul Ryan of Wisconsin greet supporters during a homecoming campaign rally Sunday in Waukesha, Wis. Justin Sullivan/Getty Images hide caption

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