Supporters listen to Republican presidential candidate Rick Santorum at The Ravine in Bellevue, Wis., on March 24. Some voters in the state are complaining about a barrage of negative ads in advance of Tuesday's primary. Darren Hauck /Reuters /Landov hide caption

itoggle caption Darren Hauck /Reuters /Landov

Sen. Marco Rubio, R-Fla., talks at the Hispanic Leadership Network in Miami on Jan. 27. Alan Diaz/AP hide caption

itoggle caption Alan Diaz/AP

Former House Speaker Newt Gingrich, who wants to the the 2012 Republican presidential nominee, during a campaign event in Camp Hill, Pa., on Saturday (March 24, 2012). Jessica Kourkounis/Getty Images hide caption

itoggle caption Jessica Kourkounis/Getty Images

Republican presidential candidate Rick Santorum addresses supporters at a campaign rally in Fond du Lac, Wis., on Sunday. Jae C. Hong/AP hide caption

itoggle caption Jae C. Hong/AP