Edwards' endorsement on Wednesday deals a sharp blow to Clinton, a day after she sought to convince top Democrats that her 2-1 victory over Obama in West Virginia on the strength of working-class voters was evidence that her campaign still had signs of life despite her rival's largely insurmountable delegate lead.

Edwards, who had based much of his candidacy on supporting the working-class voters that Obama hopes to woo, made a surprise appearance with the Democratic front-runner in Grand Rapids, Michigan, a critical general election battleground state. Obama has shrugged off his largely symbolic loss to Clinton, and turned his attention to a general election matchup against Republican John McCain.

Edwards said Obama "stands with me" in a fight to cut poverty in half within 10 years. Obama devoted his speech to one of his guest's favourite topic, fighting poverty. In America, he said, "you should never be homeless, you should never be hungry."

As president, he vowed to "lift up every American out of poverty."

Earlier, Obama tried to reach out to Michigan workers, promising to pump billions of dollars into efforts to keep manufacturing jobs from being shipped overseas.