Rick Santorum on the campaign trail

Rick Santorum, the former two-term senator from Pennsylvania, was known during his tenure for being one of the more socially-conservative members of Congress. And while he started out the 2012 presidential campaign as a lesser-known candidate, Santorum pulled off a stunning come-from-behind victory on the January 3 Iowa caucuses. Since then, he's gained momentum -- and has become Mitt Romney's primary rival in the fight for the nomination.

Rick Santorum, accompanied by his daughter Elizabeth Santorum and son Daniel Santorum, speaks in front of the Supreme Court in Washington, D.C., March 26, 2012, as the court began three days of arguments on the health care law signed by President Obama.

Dusty Compton,AP Photo/The Tuscaloosa News

Republican presidential candidate Rick Santorum greets supporters at the Original Dreamland BBQ in Tuscaloosa, Ala., March 12, 2012.

Rick Santorum makes a campaign stop at McElroy's On the Bayou, March 12, 2012, in Ocean Springs, Miss.

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum speaks befoe the American Israel Public Affairs Committee (AIPAC), in Washington, March 6, 2012.

Rick Santorum gives a speech for supporters gathered during a "Rally for Rick" at Atlantic Aviation on March 1, 2012 in Atlanta, Georgia.

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum greets diners at New Beginnings Restaurant in Kentwood, Mich., Feb. 28, 2012.

Santorum puts on a cowboy hat after meeting with supporters at a rally Feb. 8, 2012, in Plano, Texas.

Santorum speaks as his wife Karen, right, listens during a primary night watch party Tuesday, Feb. 7, 2012, in St. Charles, Mo. Santorum won all three nominating contests on Feb. 7, though no binding delegates were conferred in the process.

Republican presidential candidate former Pennsylvania Sen. Rick Santorum speaks at the First Baptist Church in Naples, Fla., Jan. 25, 2012.

Santorum, speaks at a campaign rally in Coral Springs, Fla., Jan. 22, 2012.

Santorum, left, speaks as former Massachusetts Gov. Mitt Romney listens at the South Carolina Republican presidential candidate debate in Myrtle Beach, S.C., Jan. 16, 2012.

Santorum meets patrons of Tommy's Ham House Jan. 14, 2012, during a campaign event in Greenville, S.C.

Santorum, R-Pa., center left, greets people outside the Homestead Grocery and Deli Jan. 7, 2012, during a campaign stop in Amherst, N.H.

Santorum (R-PA) speaks at a town hall meeting as it is being broadcast on C-SPAN December 30, 2011 in Marshalltown, Iowa.

Santorum poses for a picture while hosting a Pinstripe Bowl watch party at Buffalo Wild Wings Grill and Bar on December 30, 2011 in Ames, Iowa.

Santorum greets supporters after giving his speech at the Stoney Creek Inn on January 3, 2012 in Johnston, Iowa. Mitt Romney bested Santorum by just eight votes in the Iowa caucuses.

Santorum, center, joined by wife Karen, waves to supporters at the finish of his post-caucus speech in Iowa on Jan. 3, 2012.

Rick Santorum looks over Iowa Republican presidential caucus returns with campaign staffers in his hotel room Jan. 3, 2012, in Johnston, Iowa.

Santorum shakes hands during a meet and greet campaign stop at Pizza Ranch, Jan. 2, 2012, in Altoona, Iowa.

Republican presidential candidate former U.S. Senator Rick Santorum (R-PA) speaks at the Smokey Row Coffeehouse Dec. 31, 2011 in Oskaloosa, Iowa.

Santorum speaks at a town hall meeting as it is being broadcast on C-SPAN Dec. 30, 2011 in Marshalltown, Iowa.

Santorum poses for a picture while hosting a Pinstripe Bowl watch party at Buffalo Wild Wings Grill and Bar on Dec. 30, 2011 in Ames, Iowa.

Santorum waits to be introduced at a campaign stop at the CASI Center for Active Seniors on December 29, 2011 in Davenport, Iowa.

Santorum takes aim at a pheasant at Doc's Hunt Club in Adel, Iowa, Monday, Dec. 26, 2011.

Republican presidential candidate and former U.S .Sen. Rick Santorum, R-Pa., addresses the Values Voter Summit, hosted by Family Research Council Action (FRC Action), Oct. 7, 2011 in Washington.

Republican presidential candidates Newt Gingrich, left, and Rick Santorum shake hands before the start of a presidential debate sponsored by CNN and The Tea Party Express at the Florida state fairgrounds Sept. 12, 2011, in Tampa, Fla.

Rick Santorum, former Senator from Pennsylvania and Republican presidential hopeful, gets a hug from Leslie Nussle and her daughter Sarah during a campaign stop at the Pizza Ranch on Aug. 9, 2011, in Manchester, Iowa.

Republican presidential candidate and former Senator Rick Santorum and his wife, Karen, follow a bagpiper while greeting people outside the Hilton Coliseum, where Iowans will vote in the Iowa Straw Poll at Iowa State University Aug. 13, 2011, in Ames, Iowa.

Former President George W. Bush stands with Santorum at Pittsburgh International Airport Air Reserve Station in Pittsburgh, Pa., March 24, 2006.

Santorum concedes his Pennsylvania senatorial race to democrat Bob Casey as his daughter Sarah Maria cries while standing next to mother Karen at the Omni William Penn Hotel on Nov. 7, 2006, in Pittsburgh, Pennsylvania.

Santorum waits on deck at the 44th Annual Congressional Baseball Game on June 23, 2005, at RFK Stadium in Washington. The Democrats played against the Republicans in a charity game benefiting the Washington Literacy Council and the Boys & Girls Clubs of Greater Washington.

Santorum speaks to people who came to protest the possible closure of bases after he testified at the Base Realignment and Closure Commission hearing July 7, 2005 in Washington.

Santorum receives a call at his campaign headquarters in the morning of Nov. 7, 2006, outside Pittsburgh, Pa. Santorum made last-minute efforts to rally voters in his race against his opponent, Democrat Bob Casey.

Santorum is greeted by a supporter during the Pulaski Day Parade Oct. 1, 2006 in Philadelphia, Pa.

Santorum, who served as chairman of the Senate Republican Conference, meets with Supreme Court nominee Judge Samuel Alito in the senator's office on Capitol Hill Nov. 18, 2005 in Washington.

Alex Wong/Getty Images for Meet the Press

Santorum and Bob Casey, participate in a debate on "Meet the Press" Sept. 3, 2006 at the NBC studios in Washington. (Casey ultimately defeated Santorum in the contest.)

Santorum and former Pennsylvania Governor Ed Rendell speak at a news conference along with local politicians at the Delaware Valley Historical Airplane Association Museum on May 13, 2005.

Santorum works on his impeachment vote statement with an unidentified aide on Feb. 11, 1999 in the U.S. Capitol during the impeachment trial of former President Bill Clinton.