Rick Santorum (from left), Newt Gingrich and Mitt Romney debate Thursday night in Jacksonville, Fla. Joe Raedle/Getty Images hide caption

itoggle caption Joe Raedle/Getty Images

Republican presidential candidates Rick Santorum (left) and Mitt Romney during a debate in South Carolina on Monday. Charles Dharapa/pool/Getty Images hide caption

itoggle caption Charles Dharapa/pool/Getty Images

Rick Santorum (right) was put on one end of the candidates, Jon Huntsman (far left) on the other during a Nov. 12 televised debate in South Carolina. During the debate, Huntsman complained about being "a little lonely over here in Siberia," and Santorum responded: "Tell me about it." Paul J. Richards/AFP/Getty Images hide caption

itoggle caption Paul J. Richards/AFP/Getty Images

Former Pennsylvania Sen. Rick Santorum talks with a customer while surrounded by news crews as he pays a visit to customers at the Tilt 'n Diner in Tilton, NH, on Jan. 5. Rod Lamkey Jr./The Washington Times /Landov hide caption

itoggle caption Rod Lamkey Jr./The Washington Times /Landov

Rick Santorum receives a call at his campaign headquarters during his Senate re-election bid in 2006. The former senator was attempting to keep his Pennsylvania Senate seat, which he later lost to Democrat Bob Casey Jr. Jeff Swensen/Getty Images hide caption

itoggle caption Jeff Swensen/Getty Images

Republican presidential candidate Rick Santorum arrives at the Grappone Conference Center in Concord, N.H., to speak to students from around the country, in an event organized by the New England College. John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

Then-Sen. Rick Santorum is interviewed after a debate with his Democratic challenger, Bob Casey, in 2006. Santorum later lost the Senate seat to Casey. Alex Wong/Getty Images for Meet the Press hide caption

itoggle caption Alex Wong/Getty Images for Meet the Press

Susan Carroll of Atkinson, N.H., reacts to news of a Santorum lead at a caucus-watching party at Santorum's New Hampshire campaign headquarters in Bedford. Carroll is the Santorum campaign's Tea Party liaison for the state and describes herself as the owner of a small business that offers "artistic services." John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

Republican presidential candidate Mitt Romney hugs his wife, Ann, in Des Moines, Iowa. Win McNamee/Getty Images hide caption

itoggle caption Win McNamee/Getty Images

Republican presidential candidate Rick Santorum points to a television showing his campaign stop on live at the Daily Grind coffee shop in Sioux City, Iowa, Sunday. Charlie Neibergall/AP hide caption

itoggle caption Charlie Neibergall/AP