The former US Democratic presidential contender, John Edwards, has endorsed Barack Obama's bid for the White House.
Speaking alongside the senator at a rally in Michigan, Mr Edwards said Mr Obama was the one man who could make the lasting change that America needed.
John Edwards dropped out of the presidential race in January.
Correspondents say his endorsement will damage Mr Obama's rival, Sen Hillary Clinton, despite her victory in the
West Virginia primary on Tuesday.
"The Democratic voters in America have made their choice and so have I," John Edwards told a crowd of cheering supporters in Grand Rapids, Michigan.
"There is one man who knows and understands that this is a time for bold leadership... 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."
Mr Edwards spoke about the need to help the poor and provide better health care, and announced that Mr Obama had backed his new anti-poverty initiative, which aims to reduce poverty in the United States by half within 10 years.
Earlier in the speech he paid tribute to Hillary Clinton's tenacity, strength and determination, and said she cared deeply about the working people of America.
Correspondents say the timing of the announcement is significant, on a day when Hillary Clinton has been celebrating a big win in West Virginia which was largely down to her support among poorer white voters - a constituency with which John Edwards is most closely identified.
'Far from over'
Both Senators Obama and Clinton had been heavily courting Mr Edwards' endorsement, as soon as he had stood down from the race in January.
But as news of the announcement broke, Mrs Clinton's campaign team dismissed its significance.
Hillary Clinton had a big win in West Virginia on Tuesday
"We respect John Edwards, but as the voters of West Virginia showed last night, this thing is far from over," Clinton campaign chairman, Terry McAuliffe, said in a statement.
So far in the battle for the Democratic presidential nomination, Mr Obama has won more of the delegates who will choose the party's nominee at the Democratic National Convention this summer.
He has also raised more money than Mrs Clinton.
The Clinton campaign has been hoping that she could pick up more endorsements than Mr Obama from the so-called "super-delegates", who get an automatic vote at the convention by virtue of holding a senior position in the party.
The BBC's Justin Webb says that with the high-profile endorsement of Mr Edwards, Mrs Clinton's already difficult case has just become even trickier to make.
According to a tally by the Associated Press, Barack Obama has a total of 1,887 delegates, leaving him just 139 delegates short of the 2,026 needed to clinch the nomination, while Hillary Clinton has 1,718 delegates.
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