Herman Cain (right) speaks as Texas Gov. Rick Perry looks on during a GOP presidential debate at Dartmouth College on Oct. 11.

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Rep. Michele Bachmann greets supporters after Tuesday's debate in New Hampshire. She saw her political fortunes rise earlier in the summer but has since fallen back in the polls.

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The type of atomic bomb that was used in Japan in World War II, known as the "Fat Man," is shown here in a 1960 photo released by the U.S. government. Liberals and conservatives are gathering at the Ronald Reagan Presidential Library on Tuesday to call for efforts to eliminate nuclear weapons.

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Obama is surrounded by former presidents in the Oval Office in 2009. Two of his predecessors — George W. Bush and Bill Clinton — won two terms, while two others — George H.W. Bush and Jimmy Carter — left office after just one.

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Former Utah Gov. Jon Huntsman (right) and Texas Gov. Rick Perry confer during a break in the Sept. 12 presidential debate in Tampa. Huntsman first objected to Perry's use of the word "treasonous" then later used it himself.

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Fans of the Tampa Bay Rays celebrate during the singing of "God Bless America" during the game against the Boston Red Sox at Tropicana Field on Sept. 11 in St. Petersburg, Fla. J. Meric/Getty Images hide caption

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In virtually every county, there is a Walmart open every hour of every day, and every one of those Walmarts is being visited by 37,000 people a week — that's 220 people an hour, in every Walmart, every hour of the day. Here, a Walmart worker pulls carts at a store in Pittsburg, Calif. Paul Sakuma/AP hide caption

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An Amtrak Police K-9 watches while commuters depart for rush hour during "Operation Railsafe" at Union Station in Washington, D.C., last October. Brendan Smialowski/Getty Images hide caption

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One of the earliest accounts of a surprise attack comes from Greek mythology: the Trojan Horse. Rischgitz/Hulton Archive/Getty Images hide caption

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In the East, they'll sell you the Brooklyn Bridge. In the West, it's the Golden Gate. Saul Loeb and Gabriel Bouys/AFP/Getty Images hide caption

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