Republican presidential candidate Mitt Romney speaks at a town hall meeting at Eagle Manufacturing Corp. in Shelby Township, Mich., on Tuesday. Gerald Herbert/AP hide caption

itoggle caption Gerald Herbert/AP

Bill Burton, shown during a news briefing at the White House in January, is now with pro-Obama superPAC Priorities USA Action. He says the superPAC is "careful to make sure that we are in compliance with the rules." J. Scott Applewhite/AP hide caption

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Sen. Charles Schumer, D-N.Y. (left), and Sen. Al Franken, D-Minn., discuss superPACs Wednesday. J. Scott Applewhite/AP hide caption

itoggle caption J. Scott Applewhite/AP

Millions of dollars have been spent on television ads during this campaign cycle. Chip Somodevilla/Getty Images hide caption

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Republican presidential candidate Newt Gingrich speaks Thursday during a Lakeside Inn Tea Party rally in Mount Dora, Fla. Joe Raedle/Getty Images hide caption

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Casino owner Sheldon Adelson and his wife, Miriam, at the Marina Bay Sands complex in Singapore in June. The Adelsons have donated $5 million each to the pro-Gingrich superPAC Winning Our Future. Roslan Rahman/AFP/Getty Images hide caption

itoggle caption Roslan Rahman/AFP/Getty Images

Dozens of televisions display a political advertisement with the image of GOP presidential hopeful Newt Gingrich at a store in Urbandale, Iowa, on Dec. 27. Republican candidates and their superPACs have spent millions on television and radio ads. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images