Mr Clinton's relationship with Mr Obama has been "frosty"
Former US President Bill Clinton has announced for the first time his support of fellow Democrat Barack Obama's bid for the White House.
Mr Clinton's wife Hillary was Mr Obama's biggest rival for the party nomination, and he was often critical of Mr Obama on the campaign trail.
Mr Clinton's spokesman said he was committed to working for an Obama win.
Mr Obama has asked his fundraisers to help pay $10m (£5m) of Mrs Clinton's debts from her failed campaign.
The Illinois senator asked them to help settle the sum - almost half the total of the former first lady's debts - in a teleconference on Tuesday.
Mr Obama and Mrs Clinton, a New York senator, are to hold a joint rally on Friday, but Mr Clinton will be in Europe and will not attend.
"President Clinton is obviously committed to doing whatever he can and is asked to do to ensure Senator Obama is the next president of the United States," said spokesman Matt McKenna.
The Obama campaign welcomed Mr Clinton's declaration of support, saying: "A unified Democratic Party is going to be a powerful force for change this year and we're confident President Clinton will play a big role in that."
BBC Washington correspondent James Coomarasamy says Mr Clinton remains a hugely popular draw for Democrats and could help Senator Obama, particularly in those working class areas where he has found it hardest to connect.
However, our correspondent adds, while the Obama campaign says it is confident Mr Clinton will play a role in unifying the Democratic party, relations between the two camps remain strained.
US media have described the mood between Mr Clinton and Mr Obama as tense since Mr Obama beat Mrs Clinton in the race for the Democratic presidential nomination.
The Associated Press news agency reports that the two men have not spoken in the aftermath of the heated and drawn-out campaign.
During the primary battle between Mr Obama and Mrs Clinton, Mr Clinton made headlines when he described Mr Obama's record of opposition to the Iraq war as a "fairy-tale", and when he accused Mr Obama of "playing the race card".
Observers say Mr Obama is making efforts to heal the divisions within the party and win over Democrats who supported Mrs Clinton in the primaries.
To this end, the Obama-Clinton rally on Friday will be held in the town of Unity, New Hampshire, where Mr Obama and Mrs Clinton each received 107 votes in the New Hampshire primary.
Meanwhile, Republican presidential candidate John McCain was in Santa Barbara, California, on Tuesday for an energy debate alongside the state's governor, Arnold Schwarzenegger.
Expanding on his ideas to address the nation's energy crisis, Mr McCain pledged to switch to green technology for official vehicles and make federal office buildings more energy efficient.