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.
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 Obama would go on.
Santorum made no mention of Romney, and stressed that he had gone farther than anyone expected, competing "against all odds".
The 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 he had nevertheless vowed to stay in the hunt, and hoped 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 have said Santorum has been pressing ahead with his 2012 campaign, despite Romney's virtual lock on the all-important delegate count, 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, could have proved fatal for Santorum should he try for the Republican nomination four years from now.
The delegate totals told the tale of Santorum's demise. 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.
Still in the race, but not considered to be factors, are former House of Representatives leader Newt Gingrich and Texas legislator Ron Paul.
