Republican presidential hopeful Rep. Michele Bachmann of Minnesota speaks to the Christian Chamber of Commerce at Christ Central Ministries in Columbia, S.C., on Monday. Brett Flashnick/AP hide caption

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While campaigning for president in 2007, Barack Obama, then a senator from Illinois, drove a bumper car with his daughter Sasha at the Iowa State Fair. Scott Olson/Getty Images hide caption

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President Obama greets patrons at Ross' Restaurant in Bettendorf, Iowa, on Tuesday. In 2008, the president's unexpected victory in the state helped propel his campaign. Saul Loeb/AFP/Getty Images hide caption

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U.S. Rep. Michele Bachmann greets Irene Blom before speaking at a Presidential Lecture Series sponsored by The Family Leader, April 11, 2011 in Pella, Iowa. Bachmann, along with Herman Cain, Newt Gingrich, Gary Johnson and Rick Santorum, are expected to drop in on a Tea Party Bus Tour making its way through Iowa. Charlie Neibergall/AP hide caption

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Then-Democratic presidential hopeful Barack Obama waves during a May 24, 2008, campaign stop in San Juan, Puerto Rico. Keeping a campaign promise, Obama returns Tuesday. Brennan Linsley/AP hide caption

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Republican presidential hopeful, former Mass. Gov. Mitt Romney meets with entrepreneurs at Bizdom U in Detroit on Thursday. Bizdom U is a business development center to help entrepreneurs start up businesses in the state. Carlos Osorio /AP hide caption

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Corn is big business in Iowa, which has usually made anything but support for ethanol subsidies off-limits for presidential hopefuls. Steve Pope/AP hide caption

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Former Massachusetts Gov. Mitt Romney focuses on a key issue — corn — in Ankeny, Iowa, on Friday, as he prepares to launch his 2012 presidential bid. Charlie Neibergall/AP hide caption

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George Romney with his wife, Lenore, and teenage son Mitt, after announcing he would seek the Republican nomination for governor of Michigan at a press conference in 1962. AP hide caption

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Republican Sen. Rand Paul of Kentucky sharply criticized the Obama administration's decision to intervene in Libya. Brendan Hoffman/Getty Images hide caption

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In a photo released by NBC News, Secretary of State Hillary Clinton and Secretary of Defense Robert Gates discuss Middle East policy on Meet the Press. William B. Plowman/AP hide caption

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Karen Wallace (right) and Meryleigh Brainerd (left), both teachers in California, join in a candlelight vigil in front of the state Capitol to express solidarity with union members in Wisconsin. Decades ago, says a labor historian, the typical unionized worker was a blue-collar male. Now, it's more likely to be a schoolteacher. Robert Durell/AP hide caption

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President Ronald Reagan campaigns for his economic policies in 1987 before a sign that reads "Truth in Spending, Americans Deserve It." Mike Sargent/AFP/Getty Images hide caption

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