Rick Santorum has ended his 2012 US presidential campaign, clearing the way for Mitt Romney to claim the Republican nomination and face incumbent Barack Obama in November's election.
Santorum, appearing with his wife and family in his home state of Pennsylvania, told supporters on Wednesday that the race was over, but that the fight to defeat Obama would go on.
But Santorum, who had been considered the standard bearer for the Republican Party's religious-right support base, made no mention of Romney, stressing instead that his challenge had gone further than anyone expected "against all odds".
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Santorum spoke with Romney ahead of his announcement, a source close to the campaign told The Associated Press news agency.
Romney congratulated Santorum on his campaign, calling him an "able and worthy competitor".
A father of seven children, Santorum's decision to quit was partly influenced by a serious illness suffered by his three-year-old daughter, Bella. She was hospitalised over the long holiday weekend with Trisomy 18, a rare genetic condition that hinders a child's development.
Santorum, 53, had been running a distant second to former Massachusetts governor Romney in the nominations race, but had nevertheless vowed to stay in the hunt, hoping a big win in his home state later this month would reignite his candidacy.
But with polls showing Romney leading Santorum in Pennsylvania, a loss in the state would mean a second crucial defeat for Santorum on home turf, after a huge loss to a Democratic challenger in his 2006 Senate re-election bid.
Experts and observers say Santorum has been pressing ahead with his 2012 campaign, despite Romney's virtually unassailable lead in delegate numbers, as a way to boost his profile as a strong alternative conservative and
lay the groundwork for a possible 2016 presidential run.
But a failure to win in his home state, after his 2006 loss, would have proved disastrous for the prospects of another Santorum presidential bid in four years time.
Romney has more than twice as many delegates as Santorum and is on pace to reach the number needed to clinch the nomination, 1,144, by early June.
But the longevity of Santorum's campaign, outlasting early pacesetters including Herman Cain and Rick Perry, drew attention to Romney's failure to connect with grassroots Republicans, many of whom consider the former Massachusetts governor to be too liberal on traditional conservative issues such as abortion and healthcare.
Still in the race, but not considered to be factors, are former House of Representatives leader Newt Gingrich and Texas legislator Ron Paul.