Newt Gingrich, the fiery former congressional leader, scored a huge win over Mitt Romney in South Carolina's Republican primary, upending the race to choose a challenger to President Barack Obama.
The commanding victory allows Gingrich to claim the mantle as the alternative to Romney, the longtime frontrunner who has failed to persuade many Republicans that he is an authentic conservative.
Romney, a former Massachusetts governor, had looked for a South Carolina victory to make him the all-but-inevitable Republican nominee. Instead, the Gingrich victory raises the prospect the race could drag out for months.
Romney had benefited in earlier contests from having the conservative vote divided among Gingrich, Rick Santorum and Rick Perry. But with Perry, the Texas governor, dropping out Thursday and Santorum finishing a distant third Saturday, Gingrich will look to coalesce the conservative vote ahead of the next contest, the 31 January primary in the big battleground state of Florida.
If the race boils down to Gingrich and Romney, it would be a fire-and-ice matchup: Gingrich, a passionate, spontaneous, self-styled visionary; Romney, a clean-cut, cool, methodical former CEO.
Obama and his fellow Democrats would relish a tough, drawn-out Republican primary fight that ultimately weakens the nominee. Democrats see Romney as the toughest potential opponent for Obama, whose reelection prospects have been damaged by the sluggish US economy.
Gingrich's campaign still faces many obstacles. While South Carolina's conservative electorate was a good fit for Gingrich, who is from neighboring Georgia, other states could be more difficult. Gingrich lacks Romney's money and organisation. And he has more baggage than other contenders, including admitted extramarital affairs, two divorces, an ethics reprimand when he was speaker of the House of Representatives, and questions about his post-Congress business dealings.
Still, he has been underestimated before. Twice his campaign was considered effectively dead, yet he has rebounded to win what many consider the most important of the first three primaries. And while much of the Republican establishment is wary of him, his bashing of "elites" can resonate with the Republican base.
In his victory speech, Gingrich urged supporters to donate and get involved.
"We don't have the kind of money that at least one of the candidates has," he said in a reference to Romney. "And we proved here in South Carolina that people powered with the right ideas beats big money."
Returns from 95 per cent of the state's precincts showed Gingrich with 41 per cent of the vote to 27 per cent for Romney. Santorum, a former senator, was winning 17 per cent and Texas congressman Ron Paul 13 per cent.
Santorum has been unable to build on his narrow victory in the first contest, the 3 January Iowa caucuses. He said he will stay in the race, but lacking money and momentum, his prospects appeared dim.
Paul's weak showing was expected. While he has drawn many supporters to his libertarian, small-government message, his call to withdraw US troops from around the world was a tough sell in a state dotted with military installations. He remains a longshot for the nomination.
Romney, who easily won the New Hampshire primary, has cast himself the candidate most likely to beat Obama. But South Carolina voters weren't persuaded. Voters who said they cared most about picking a candidate who could defeat Obama favoured Gingrich, according to an exit poll conducted by Edison Research for The Associated Press and US television networks.
South Carolina, a southern state with one of the most conservative electorates, is difficult territory for a former northeastern governor like Romney, especially given his position changes on important social issues like abortion. Some of the state's evangelical Christians may be uncomfortable with his Mormon faith. In the 2008 primary, he finished fourth.
Romney had a wide lead in South Carolina opinion polls following his victory in the 10 January New Hampshire primary. But he had a difficult week as Iowa, where he had initially been declared a winner, was awarded to Santorum. Romney, a multimillionaire former venture capitalist, also responded awkwardly to questions about releasing his income tax returns and about his investments in the Cayman Islands.
Gingrich, meanwhile, rebounded strongly from weak showings in Iowa and New Hampshire, boosted by strong debate performances.
He didn't flinch when ex-wife Marianne said in an interview on ABC television that he had been unfaithful for years before their divorce, and asked him for an open marriage so he could keep his mistress, who is now his wife.
Asked about the accusation in a debate Thursday, he unleashed an attack on ABC and debate host CNN and accused the "liberal news media" of trying to help Obama by attacking Republicans. His ex-wife's account, he said, was untrue.
With his South Carolina victory, Gingrich won at least 23 delegates to the Republican National Convention in August. Two more are to be awarded.
The numbers are a small fraction of 1,144 delegates needed to win the nomination. But South Carolina was always more about political momentum than delegates.
As the first primary in a southern state, South Carolina has been a proving ground for Republican candidates. Since 1980, every Republican contender who won the primary has gone on to capture the party's nomination.
Nearly 600,000 voters turned out, according to an AP estimate. That eclipses the previous record turnout for the primary in 2000, when George W. Bush defeated McCain.