Newt Gingrich stormed to a shock victory over Mitt Romney in the South Carolina primary today, scrambling the race to choose a Republican challenger to President Barack Obama.
The win marks a dramatic turnaround from Mr Gingrich's earlier poor showing in the first two contests of the party's presidential race.
It sets up the likelihood that the contest, which Mr Romney once seemed poised to wrap up quickly, could drag on for months.
It also puts Mr Gingrich in a position to establish himself as the true conservative alternative to Mr Romney, who some Republicans see as too moderate.
Romney has benefited by having the conservative vote divided among Mr Gingrich, former senator Rick Santorum and Texas governor Rick Perry, who dropped out on Thursday.
An exit poll conducted for The Associated Press and US television networks showed Mr Santorum and congressman Ron Paul were far behind.
Gingrich led by a wide margin among the state's heavy population of conservatives, born-again Christians and supporters of the small-government tea party movement.
The poll by Edison Research involved interviews with 1,577 voters and had a margin of sampling error of plus or minus four percentage points.
Mr Gingrich also won the support of voters who said they cared most about picking a candidate who could defeat Mr Obama. Mr Romney has made the claim that he is the most electable candidate a key part of his campaign.
Mr Romney, the former governor of Massachusetts, once appeared poised to sweep the first three contests in the Republican race and become the all-but-inevitable nominee.
But what had appeared to be a narrow victory in the first contest, the Iowa caucuses, was later determined to by a narrow loss to Mr Santorum. He easily won the New Hampshire primary, but lost what had been a substantial lead in pre-election polls in South Carolina.
The conservative southern state has long been difficult territory for the former governor of the liberal north-eastern state of Massachusetts. He finished fourth there behind the eventual nominee, Senator John McCain, in the 2008 race.
Mr Gingrich pressed ahead despite poor showings in Iowa and New Hampshire, boosted by strong debate performances and shrugging off allegations by an ex-wife that he had once asked her for an open marriage so he could keep his mistress.
Mr Santorum's poor showing is a further sign he lacks the organisation and money to build on his Iowa victory. If he withdraws at some point, that could benefit Gingrich.
Mr Paul had not been expected to do well in South Carolina. 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 and home to many veterans.
The vote was the climax to a tumultuous week in which Mr Romney was stripped of his Iowa triumph and contended with uncomfortable questions about his finances. Mr Perry and former Utah governor Jon Huntsman dropped out of the race.
Mr Romney, a multimillionaire former venture capitalist, responded awkwardly to questions about releasing his income tax returns, and about his investments in the Cayman Islands.
The Republican nominee is determined by a series of state-by-state contests to select delegates to the Republican National Convention in late August. Only 25 of the 2,286 delegates are at stake in South Carolina, but political momentum is the real prize.
As the first primary in a southern state, South Carolina has been a proving ground for Republican presidential hopefuls in recent years.
Since Ronald Reagan won in 1980, every Republican contender who won the primary has gone on to capture the party's nomination.
In all, more than 12 million dollars (£7.7m) was spent on television ads by the candidates and their allies in South Carolina, much of it on attacks designed to degrade the support of rivals.
Already, Mr Romney and a group that supports him were on the air in next-up Florida with a significant ad campaign, more than seven million dollars (£4.5m) combined to date. The state's primary is 31 January.
As Mr Gingrich's victory became apparent, he swiftly appealed to supporters for donations ahead of that contest.
"Help me deliver the knockout punch in Florida. Join our Moneybomb and donate now," he tweeted.
Mr Romney told supporters he would compete for every vote in every state.
He criticised Mr Gingrich without naming him, saying his opponent was joining in on a frontal assault on free enterprise when he castigated Mr Romney's time at a private equity firm.
Mr Romney said his party could not choose someone who had never run a business or a state.