Two weeks after his wife ended her White House bid, Mr Clinton's spokesperson said he is committed to doing whatever he can to ensure Senator Obama is the next president of the US.
Mr Obama's campaign team welcomed the endorsement, saying a unified Democratic Party will be 'a powerful force for change'.
Senator Obama and former first lady Hillary Clinton will campaign together on Friday when they will hold a joint rally.
After greeting dozens of staff and well-wishers gathered on the Senate steps, Hillary Clinton told reporters overnight that Friday would 'be a symbolic event that I hope will rally the Democratic Party behind our nominee'.
Asked about running as Senator Obama's vice presidential nominee, the New York senator said: 'You know, it is not something that I think about. This is totally Senator Obama's decision and that's the way it should be.'
And Senator Clinton, nearly three weeks after conceding and pledging an all-out joint effort against Senator John McCain, warned disaffected supporters who might be considering a protest vote for the Republican to reconsider.
'If you care about the issues I care about and the future that I outlined during my campaign, then you really have to stay with us in the Democratic Party and vote for Senator Obama to be our next president,' she said.
Poll puts Obama ahead of McCain
When the primaries were in full spate, up to one-third of Clinton supporters had said they would rather vote for Senator McCain than Senator Obama. But her message has already filtered through to those voters in recent poll findings.
A Los Angeles Times/Bloomberg survey yesterday gave Senator Obama a 12-point lead in a head-to-head contest with Senator McCain - 49% to 37%.
The great majority of Senator Clinton's voters have transferred their allegiance to Senator Obama, the poll suggested, with only 11% of the former first lady's backers planning to defect to Senator McCain.
Bill Clinton was the fiercest partisan of his wife's failed shot at the Democratic nomination, and frequently lashed out as the African-American Obama built up unstoppable momentum against the Democrats' first couple.
The former president dismissed Senator Obama's opposition to the Iraq war as a 'fairytale', and appeared to belittle Obama's triumph with black voters in January's South Carolina primary.
Senator Obama and Hillary Clinton spoke on Sunday to discuss how the new Democratic standard-bearer could liaise with Bill Clinton in future, Obama spokesman Bill Burton said.
'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,' he said.
Hillary Clinton, attending a luncheon with her Democratic Senate colleagues, was all smiles as she made a high-profile entrance at the steps of Congress, rather than going through one of Capitol Hill's discreet tunnels.
On Thursday, after addressing a conference of Latino politicians, Senator Clinton is scheduled to introduce Senator Obama to a private meeting of her leading fundraisers at a Washington hotel.
She has her own campaign debts of €14m to pay off, and some Clinton supporters want Senator Obama to help in that effort.
And Senator Obama has asked his top contributors to help Hillary Clinton retire her debt, an Obama campaign source told CNN television, ahead of their appearance in Unity.
Senator Obama did not direct members of his National Finance Committee to contribute to Clinton's campaign, the source said, but asked them to do so if they were so inclined, CNN reported.
At an event in New Mexico on Monday, Senator Obama said he wanted all American girls including his two daughters to 'truly have the same opportunities as our sons,' and said the 'extraordinary' Senator Clinton had brought that goal nearer.
The Illinois senator pledged to work with Senator Clinton on 'the issues that matter to American women and to all American families,' including healthcare, education and equal pay.
