Former US senator Rick Santorum announced today that he is suspending his presidential bid, removing the last major hurdle for Mitt Romney to claim the Republican nomination.
"We made the decision to get into this race at our kitchen table against all the odds," Mr Santorum said at a news conference in Gettysburg, Pennsylvania.
"And we made a decision over the weekend that while this presidential race is over for me, and we will suspend the campaign today, we will continue the fight."
Mr Santorum has long trailed Mr Romney in the race for the delegates needed for their party's nomination.
In a statement, Mr Romney said Santorum proved himself "an able and worthy competitor".
"I congratulate him on the campaign he ran. He has proven himself to be an important voice in our party and in the nation. We both recognise that what is most important is putting the failures of the last three years behind us and setting America back on the path to prosperity."
Mr Santorum spoke to Mr Romney today but did not announce an endorsement of the front-runner or either of the other two Republican candidates, Ron Paul and Newt Gingrich, both of whom are way behind Mr Romney in polls.
Mr Santorum (53) presented himself in the primary as the leading voice for voters motivated by their opposition to abortion rights, gay marriage and other social issues. He also said he was better suited to take on president Barack Obama because of his longstanding opposition to the health-care law that Mr Obama pushed through Congress.
Mr Romney (65) signed a Massachusetts statute that contained a mandate to purchase health care insurance coverage similar to the one included in Mr Obama's law, which the Supreme Court now is weighing on constitutional grounds.
Mr Santorum made the decision at a time when he'd halted campaign activities after the hospitalisation of his three-year- old daughter, Bella. She suffers from a rare genetic disorder known as Trisomy 18.
Hogan Gidley, his communications director, released a statement today saying that Mr Santorum's' daughter "has been discharged from the hospital and returned home earlier Monday evening."
Mr Santorum said today he made his decision after a "difficult weekend" with his sick daughter. "It did pause us to think of the role we have as parents."
Mr Santorum, who had been bolstered in the race by victories in Iowa, Mississippi and Alabama, said "we were winning in a very different way because we were touching hearts." Surrounded by supporters, he said, "it really wasn't my voice that I was out communicating. It was your voice."
In recent weeks Mr Santorum saw influential Republican backers gather around Mr Romney, including endorsements of the former Massachusetts governor by Wisconsin representative Paul Ryan, Florida Senator Marco Rubio, who is often mentioned as a vice-presidential prospect, and former president George H.W. Bush.
Mr Santorum, former House Speaker Newt Gingrich of Georgia, and Representative Ron Paul of Texas had vowed to stay in the race until Mr Romney secured the 1,144 delegates needed for the nomination.
Mr Romney's trio of victories last week in Wisconsin, Maryland and the District of Columbia accelerated his path to the nomination and intensified questions about the candidacies of his rivals.
After the latest round of voting, Mr Romney had 655 of the 1,144 delegates needed to capture the nomination, according to an Associated Press tally. Mr Santorum has 278 delegates and would need to win about three-quarters of those remaining.
Bloomberg