Edwards, the 2004 vice-presidential nominee, had been heavily courted by both Obama and rival Clinton in the past few months.


His backing could aid Obama's efforts to win the support of the white working-class voters who have flocked to Clinton in recent contests.


Appearing with Obama in Michigan, Edwards appealed to the party to "come together as Democrats" to defeat John McCain, the likely Republican candidate.


"We are here tonight because the Democratic voters have made their choice, and so have I," he said, adding that Obama "stands with me" in a fight to halve poverty within 10 years.


Edwards also praised Clinton saying "we are a stronger party" because of her involvement, and "we're going to have a stronger nominee in the fall because of her work".


But he said "there is one man who knows in his heart that it is time to create one America, not two, and that man is Barack Obama".


Advantage Obama


Obama also received support from other leading Democrats on Wednesday, widening his apparently insurmountable lead over Clinton by picking up three more superdelegates, the party leaders who can vote at the nominating convention for whichever candidate they want.


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Clinton's campaign shrugged off the endorsement, saying "we respect John Edwards, but as the voters of West Virginia showed last night, this thing is far from over".


The former first lady picked up one superdelegate on Wednesday and vowed to stay in the campaign despite struggling with a $20m debt.


"Choose who you believe will make the strongest candidate in the fall," Clinton said at her Charleston, West Virginia, rally. "The White House is won in the swing states, and I am winning the swing states."


She continues her long-shot strategy of trying to persuade the superdelegates that only she can attract the working-class votes crucial for Democratic hopes of winning the states they need to defeat McCain in November.


Obama has 1,887 delegates to Clinton's 1,718, with 2,026 needed to win the nomination.


About 250 of the nearly 800 superdelegates remain uncommitted.


But the trend of superdelegates has been overwhelmingly in Obama's favour.


He picked up more than 30 in the past week - more than the 20 delegates Clinton won in West Virginia.