Early exit polls showed Mr Edwards holding a double-digit lead over Mr Kerry, with all the other candidates trailing in single digits. Mr Edwards was also in a tight three-horse race with Mr Kerry and Wesley Clark in Oklahoma, one of seven states which held primaries and caucuses last night.

Mr Kerry remained the frontrunner with polls indicating a commanding lead in five states - Arizona, Delaware, Missouri, New Mexico and North Dakota. His victories in Iowa and New Hampshire over the last two weeks have given his campaign huge momentum, bringing in money and endorsements. Today he is expected to clinch the support of the 1.2 million member American Federation of Teachers, the country's second largest teachers' union.

But Mr Edward's victory in South Carolina keeps him firmly in contention, while denying Mr Kerry the clean sweep that would have effectively ended the race. "If I win here," Edwards said confidently in Clinton, South Carolina, "I'll be the nominee."

After his poor showing in New Hampshire, General Clark's prospects hinged on a win in Oklahoma, while Joseph Lieberman was widely expected to withdraw from the race after a poor showing across the board. He was due to address supporters in Arlington, Virginia,last night.

Howard Dean, who had campaigned little in the seven states this week, has pinned hopes of reviving his campaign in Michigan and Washington state on Saturday, and in Wisconsin on February 17. It marks a reversal of his strategy of riding early momentum, but his exhausted campaign fund gave him little choice.

In his first serious test among black voters, the only African American candidate in the race, the Reverend Al Sharpton, was still only in single digits in early exit polls.

Mr Edwards had staked everything on winning in South Carolina, spending more time in the state than any other candidate, with the exception of Mr Sharpton, and insisting he would bow out unless he won. His campaign made great play of his southern roots; he was born in the state and serves as senator in neighbouring North Carolina. "The south is not George Bush's backyard," he said. "It's my backyard, and I can beat him in my own backyard."

Mr Edwards will now focus on Virginia and Tennessee, which hold primaries next Tuesday. But to emerge as anything more than a strong vice-presidential candidate, he will have to prove he is more than a regional candidate.