U.S. Secret Service

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U.S. Secret Service agents stands watch as Republican presidential candidate Mitt Romney addresses the U.S. Hispanic Chamber of Commerce in Los Angeles on Monday, Sept. 17, 2012. (AP Photo/Charles Dharapak)

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Accompanied by U.S. Secret Service agents, Republican presidential candidate, former Massachusetts Gov. Mitt Romney greets supporters as he campaigns at Integrity Windows in Roanoke, Va., Thursday, Nov. 1, 2012. (AP Photo/Charles Dharapak)

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U.S. Secret Service agents watch as Republican presidential candidate, former Massachusetts Gov. Mitt Romney walks in a drizzle to board his campaign plane in Vandalia, Ohio, Tuesday, Oct. 30, 2012, as he travels to Tampa, Fla. for a campaign stop. (AP Photo/Charles Dharapak)

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U.S. Secret Service agents walk April 14, 2012, around the Convention Center in Cartagena, Colombia, prior to the opening ceremony of the 6th Summit of the Americas. (Associated Press)

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President Barack Obama walks with a U.S. Secret Service agent back to his vehicle after playing a private game of basketball at Fort McNair in Washington on Friday, Nov. 26, 2010. (AP Photo/Charles Dharapak)