Bowing to the inevitable, Rick Santorum quit the presidential campaign today, clearing the way for Mitt Romney to claim the Republican nomination.
Mr Santorum, appearing with his wife and family in his home state of Pennsylvania, told supporters the race for him was over, but the fight to defeat president Barack Obama would go on.
Mr Santorum made no mention of Mr Romney, and stressed that he had gone further than anyone expected, competing "against all odds".
The delegate totals told the tale of Mr Santorum's demise. Mr Romney has more than twice as many delegates as Mr Santorum and is on pace to reach the number needed to clinch the nomination - 1,144 - by early June.
Still in the race, but not considered a factor are former House Speaker Newt Gingrich and Ron Paul of Texas.
He made no mention or endorsement of Mr Romney, whom Mr Santorum had derided as an unworthy standard-bearer for the Republicans.
The former Pennsylvania senator said: "We are going to continue to go out there and fight to defeat president Barack Obama."
Mr Santorum spoke to Mr Romney ahead of his announcement, a Republican source close to the campaign said.
Mr Romney congratulated Mr Santorum on his campaign, calling him an "able and worthy competitor".
Mr Santorum had been hoping to hold out through the primary in Pennsylvania on April 24, but he decided to fold up after spending the weekend at home with his severely ill three-year-old daughter Bella.
Mr Santorum, a feisty campaigner who took everyone by surprise with his win in Iowa's lead-off caucuses, ran on his conservative credentials. But he was hobbled by a lack of money and organisation.
Mr Santorum stressed the improbable accomplishment of the past year, saying that "against all odds, we won 11 states, millions of voters, millions of votes".
He said that while Mr Romney was accumulating more delegates, "we were winning in a very different way. We were touching hearts" with his conservative message.
In a statement, Mr Romney said Mr Santorum "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."