Republican Mitt Romney addresses supporters in Boston (AP)
Mitt Romney boosted his prospects of becoming the Republican presidential nominee by winning six states including the coveted prize of Ohio on Super Tuesday.
But Mr Romney failed to land a knockout blow against his main rival, Rick Santorum, who won three contests and barely lost Ohio. Newt Gingrich kept his candidacy alive by winning his home state of Georgia.
The mixed results suggest that Mr Romney, despite padding his lead in the tally of delegates who will pick the nominee, is still unable to win over some conservatives and rally the party behind his candidacy.
With Mr Santorum and Mr Gingrich energised by their wins, the often-acrimonious race is likely to continue for weeks or months, perhaps weakening the eventual nominee and benefiting Barack Obama, whose standing in opinion polls has improved.
Mr Romney scored a home state win in Massachusetts, where he served as governor, to go with primary victories in Vermont and in Virginia - where neither Mr Santorum nor Mr Gingrich was on the ballot. He later added the Idaho and Alaska caucuses to his column. Mr Santorum won the primaries in Oklahoma and Tennessee and the North Dakota caucuses.
But the most closely watched contest was in Ohio, a heavily populated Midwestern industrial state often seen as a bellwether in presidential elections. It was a test of strength for Mr Santorum, who was a US senator from neighbouring Pennsylvania.
Mr Romney trailed much of the night but rallied near midnight. With 99% of Ohio's precincts reporting, he had 38% to Mr Santorum's 37%, an uncomfortably close margin for a candidate who had spent nearly four times as much money as his rival in the state.
But Mr Santorum is falling further behind Mr Romney in the delegate count and it is not clear how he can catch up. Moreover, Mr Gingrich, the former speaker of the House of Representatives who represented a suburban Atlanta district in Congress for two decades, insists he will stay in the race after his win in Georgia and could split the conservative vote as he vies to be Mr Romney's main rival.
At stake on Tuesday were 419 delegates, more than a third of the 1,144 delegates needed to win the nomination at the party's national convention in late August in Tampa, Florida.
Mr Romney picked up at least 212 delegates on Tuesday, Mr Santorum at least 84 and Mr Gingrich at least 72. In the overall race for delegates, Mr Romney leads with 415, Mr Santorum has 176, Mr Gingrich has 105 and Ron Paul has 47.