Republican presidential candidate Mitt Romney greets supporters in Aston, Pa., on April 23. Jessica Kourkounis/Getty Images hide caption

itoggle caption Jessica Kourkounis/Getty Images

President Obama boards Air Force One at Andrews Air Force Base in Maryland, en route to Florida, on April 10. Whenever a president runs for re-election, his travel tends to become a political issue. Cliff Owen/AP hide caption

itoggle caption Cliff Owen/AP

Mitt Romney may like to say the president is out of ideas, but Obama's re-election campaign is definitely not out of money. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

Selina Gray of Sanford, Fla., shows her sign at a rally protesting the death of Trayvon Martin, the unarmed teen shot by a neighborhood watch volunteer. Authorities have cited the state's "stand your ground" law as a reason charges have not been filed in Martin's death. Julie Fletcher/AP hide caption

itoggle caption Julie Fletcher/AP

Foster Friess introduces Rick Santorum at the 2012 CPAC Conference in Washington, D.C., on Feb. 10 wearing a sweater vest embroidered with the candidate's name. Ron Sachs/CNP DPA/Landov hide caption

itoggle caption Ron Sachs/CNP DPA/Landov

Sheldon Adelson speaks at the 2008 "Facing Tomorrow" Presidential Conference in Jerusalem. David Silverman/Getty Images hide caption

itoggle caption David Silverman/Getty Images

The superPAC Campaign for Primary Accountability is taking aim at Alabama Republican Rep. Spencer Bachus and other congressional incumbents. YouTube hide caption

itoggle caption YouTube

Republican presidential candidate Mitt Romney campaigns at a town hall meeting in Bexley, Ohio, last month. Romney won Ohio by less than 1 percent in Tuesday's primary. Gerald Herbert/AP hide caption

itoggle caption Gerald Herbert/AP

Republican presidential candidate Mitt Romney greets supporters at a town hall meeting at Taylor Winfield in Youngstown, Ohio, on Monday. Gerald Herbert/AP hide caption

itoggle caption Gerald Herbert/AP

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

itoggle caption J. Scott Applewhite/AP

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

itoggle caption Chip Somodevilla/Getty Images

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

itoggle caption Joe Raedle/Getty Images