Newt Gingrich has won the Republican presidential primary in South Carolina, beating favourite Mitt Romney and raising the possibility of a lengthy campaign to choose the man who will challenge US President Barack Obama in November.
With nearly all the votes counted, Gingrich, the former speaker of the House of Representatives, took roughly 40 per cent of the vote.
Romney, the former governor of Massachusetts, came in second in Saturday's vote with 28 per cent, US networks reported. Rick Santorum, a former senator from Pennsylvania, was in third with 17 per cent and US congressman Ron Paul in fourth with 13 per cent.
Gingrich's victory means that three different candidates have won the first three contests in the state-by-state Republican primary, reflecting a party electorate that has yet to make up its mind.
Santorum won the Iowa caucuses on January 3, and Romney won the New Hampshire primary on January 10.
Speaking at a late-night victory rally in Greenville on Saturday, Gingrich complimented his rivals before laying into Obama, whom he called a "radical" who would transform the United States into a European-style socialist state.
"If Barack Obama can get re-elected after this disaster, just think how radical he would be in a second term," Gingrich said, drawing shouts and applause.
In a speech after polls closed, Romney acknowledged Gingrich's victory but said he would fight on to the next contest, in Florida on January 31.
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Without naming Gingrich, Romney compared his Republican competitors unfavourably with Obama, and said he was the candidate who stood for his party's values of "free enterprise and free markets and consumer choice".
"Our party can't be led to victory by someone who also has never run a business and never run a state," he said. "We cannot defeat that president with a candidate who has joined in that very assault on free enterprise."
Al Jazeera's Alan Fisher, reporting from Columbia, the South Carolina capital, said Gingrich had made a "remarkable turnaround" after trailing Romney by nine percentage points in recent opinion polls.
In response, Romney appeared to change tactics, "going at [Gingrich] directly for really the first time in this campaign", our correspondent said.
But Romney's attack along free enterprise lines might not stick in an election year when most voters are worried about the economy, Jason Johnson, a political analyst, told Al Jazeera.
"That argument is never going to work ... you've got a nation where people are angry about bank bailouts; free enterprise is not the argument you want to be making in 2012," he said.
Ryan Grim, the Washington DC bureau chief for the liberal-leaning Huffington Post website, said Romney's loss in South Carolina had wiped away the "sense of inevitability" surrounding his candidacy.
"People in his own party, even people who are voting for him don't like him ... they don't identify with him, they think his jokes fall flat, they think he's an awkward guy," Grim said. "The only thing that was driving him was the weakness of these other candidates."
'Open marriage' claim
Romney's second-place showing in South Carolina came after he slipped in the polls in recent days under criticism over his reluctance to release personal tax information and his admission that he pays a much lower tax rate than many Americans.
Romney, the former chief of private equity firm Bain Capital, is one of the wealthiest men to run for president.
The Gingrich victory followed disappointing finishes in the first contests in Iowa and New Hampshire and a last-minute interview with one of his former wives, who said Gingrich had requested an "open marriage", meaning her consent to sleep with other partners.
But the interview seemed only to improve Gingrich's standing in the polls, and he used the story, which he called false, to berate what he called the "elite media" in a televised debate.
The next contest on the calendar is the important Florida primary on January 31. It will be the largest state yet in the
nomination battle and one that will require the candidates to spend heavily on advertising in many cities.