Republican Presidential Endorsements of 2012 -- PICTURES

Rep. Paul Ryan, R-Wis., threw his weight behind Mitt Romney on Friday.

PHOTO: AP Photo/Paul Sakuma

Former President George H.W. Bush endorsed Mitt Romney on Thursday.

PHOTO: AP Photo/Pat Sullivan

Former Florida Gov. Jeb Bush endorsed Mitt Romney on March 20. They're pictured here in 2005.

PHOTO: AP Photo/Steven Senne

Donald Trump endorses Mitt Romney for President on Feb. 2, 2012, in Las Vegas.

PHOTO: GERALD HERBERT/AP

Herman Cain officially endorsed Newt Gingrich for president on Jan. 28, 2012, prior to the Florida GOP primary.

PHOTO: MATT ROURKE/AP

Former GOP presidential candidate Herman Cain sings to the crowd during comedian Stephen Colbert's "Rock Me Like a Herman Cain South Cain-olina Primary Rally" at the College of Charleston in Charleston, S.C., on Friday, Jan. 20, 2012, a day after endorsing "the people" for the presidency.

PHOTO: BRUCE SMITH/AP

Former Utah Gov. Jon Huntsman announces his withdrawal from the race and endorses Mitt Romney for president on Monday, Jan. 16, 2012, in Myrtle Beach, S.C.

PHOTO: DAVID GOLDMAN/AP

Texas Gov. Rick Perry pauses during a news conference in North Charleston, S.C., on Thursday, Jan. 19, 2012, where he announced he was suspending his campaign and endorsing Newt Gingrich.

PHOTO: DAVID GOLDMAN/AP

Former Massachusetts Gov. Mitt Romney and Sen. John McCain, R-Ariz., step off a charter plane in Pensacola, Fla., on Friday, Jan. 27, 2012. Sen. McCain endorsed Romney on Jan. 4.

PHOTO: CHARLES DHARAPAK/AP

Former Massachusetts Gov. Mitt Romney campaigns with New Jersey Gov. Chris Christie in Exeter, N.H., on Jan. 8, 2012. Christie officially endorsed Romney for president on Oct. 11, 2011.

PHOTO: CHARLES DHARAPAK/AP