Results, with counting almost complete: Obama 55 per cent - Clinton 27 per cent - Edwards 18 per cent

Barack Obama has crushed Hillary Clinton in the South Carolina primary, beating her by a margin of two to one, and throwing the increasingly bitter battle for the Democratic nomination into fresh doubt.

Exit polls suggested he had received four in five black votes - and a quarter of those cast by white voters. But he also beat his rival in every age or income group on Saturday.

News organisations declared Mr Obama the winner just seconds after voting stopped at 7pm Eastern Time.

Mrs Clinton was a long way back in second place, ahead of John Edwards, a "native son" of South Carolina who won the state's primary four years ago.

Around half the turnout in South Carolina on Saturday was black. According to exit polls, black women turned out in particularly large numbers and also gave overwhelming backing to Mr Obama - suggesting race had trumped gender in this state.

Mrs Clinton won around a fifth of the black vote. Surveys suggested she had shared white support more or less equally with Mr Edwards, with around 40 per cent each, compared to 25 per cent for Mr Obama.

But in his victory speech, Mr Obama repeatedly tried to turn away from the racial themes which has cast a long shadow over this primary. "The choice in this election is not about regions or religions or genders. It's not about rich versus poor, young versus old and it's not about black versus white. It's about the past versus the future."

His supporters chanted "race doesn't matter" and "Yes, We Can!"

The Democratic candidates will now focus more than ever on "Super Tuesday" - February 5 - when many of America's biggest states will make their choice for the presidential nominee.

Mr Edwards hinted the contest could continue all the way to the Convention in Denver this August, as he promised to remain in the race 'for the long haul".

Mr Obama will on Sunday be in Georgia, while Mrs Clinton travelled to Tennessee even before conceding defeat. "I can't imagine anywhere I would rather be than here in Nashville as we kick off the next 10 days," she said later.

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