Rick Santorum has dropped out of the race for the US Republican presidential nomination, leaving the field clear for Mitt Romney as the party's likely choice to challenge President Barack Obama.
The former Pennsylvania senator made the announcement in his home state, two weeks before the Republican primary vote there. Mr Santorum was trailing Mr Romney in polls in Pennsylvania and had said the state was a must-win to keep his candidacy alive.
"This race is over for me," Mr Santorum told supporters of his improbable campaign, saying he and his family had made the decision over the weekend after his three-year-old daughter's latest hospital admission.
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Mr Santorum was a favourite of the most conservative Republican voters, but Mr Romney, the former Massachusetts governor, had accumulated a huge lead in delegates to the party's national convention in August.
Mr Santorum pointedly made no mention or endorsement of Mr Romney, whom he had derided as an unworthy standard-bearer for the Republicans. Nevertheless, Mr Romney congratulated Mr Santorum on his campaign, calling him an "able and worthy competitor".
Mr Romney's conservative credentials are suspect among the conservative Republican base, but Mr Santorum's inability to put together primary election victories in key states - especially in the industrial Midwest - appeared to have convinced most voters that Mr Romney's nomination was inevitable.
Mr Santorum was the only Republican in what started out as a crowded field who was able to create a sustained challenge to Mr Romney, who is making his second run for the nomination.
Mr Santorum was to have returned to the campaign trail Tuesday after his daughter Bella was released from hospital. She suffers from a rare genetic condition and was taken to hospital on Friday as her father began a brief break from campaigning. She was discharged on Monday night.
Mr Santorum said her latest crisis led him and his family to decide against continuing in the race.
Five states, including Pennsylvania, hold primaries on April 24