'The Democratic voters of America have made their choice and so have I,' Mr Edwards said, at a raucous Obama rally in Grand Rapids, Michigan. -- PHOTO: REUTERS

'The Democratic voters of America have made their choice and so have I,' Mr Edwards said, at a raucous Obama rally in Grand Rapids, Michigan.

But Mr Edwards also praised beleaguered Democratic hopeful Hillary Clinton, who has vowed to stay in the race, despite Mr Obama's overwhelming mathematical advantage, though the crowd initially booed when he mentioned her name.

'It is very, very hard to get up every day and do what she has done, it is hard to get out there and fight and speak up when the odds turn against you,' the former North Carolina senator said of Mrs Clinton.

Mr Edwards, the 2004 vice presidential nominee, dropped out of the Democratic race in January and was heavily courted by both Mr Obama and rival Hillary Clinton in the past few months. He made the endorsement at a Grand Rapids rally.

The move comes the day after Mr Obama was blown out by 41 points by Mrs Clinton in a nominating contest in West Virginia, but the loss was not enough to put much of a dent in his lead in the Democratic race.

VIDEO Edwards endorses Obama (1:12)

He retains an almost unassailable advantage in delegates who will select the Democratic nominee at the party convention in August. He gained the support on Wednesday of three more superdelegates, who are free to back any candidate, and the abortion rights group NARAL Pro-Choice America.

Mr Obama spent the day in Michigan looking past the race with Mrs Clinton to focus on a likely November match-up with Republican John McCain.

He said the Arizona senator 'is not offering new solutions or economic policies that are different from what George Bush has given us for eight long years.'

Mrs Clinton returned to Washington for a round of media interviews and meetings with top donors.

She promised to push on through the last five contests in the hope her showing will bolster her argument that she is the Democrat with the best chance to beat Mr McCain in November.

'I'm going to keep fighting until every last American has a chance to be heard, and as we learned last night in West Virginia, I know we can win,' the New York senator and former first lady said in an e-mail plea for donations.

Her campaign is US$20 million (S$27.6 million) in debt but her campaign chairman, Mr Terry McAuliffe, said she had the resources to compete with Mr Obama and described her donors as 'very excited, ready to go and ready to help.' Mrs Clinton added one superdelegate endorsement on Wednesday. -- REUTERS, AFP