Barack Obama became the official presidential nominee for the Democratic party in all but name last night and paid tribute to Hillary Clinton for her "valiant campaign" as she formally bowed out of the race.
He said his former rival had "shattered barriers on behalf of my daughters and women everywhere".
Obama's comments came after Mrs Clinton formally abandoned her bid for the nomination at a rally in Washington.
She said Mr Obama had proved his "grace and grit", and she urged her supporters to put their energy into electing him.
Mr Obama is expected to face the Republican presumptive nominee, John McCain, in November's presidential election.
In a statement, Mr Obama said he was "thrilled" to have Mrs Clinton's endorsement.
He credited her with reaching out to many American voters and making him a stronger candidate.
"She inspired millions with her strength, courage and unyielding commitment to the cause of working Americans."
Mr Obama also said Mrs Clinton's presence on the American political scene would continue.
"No one knows better than Senator Clinton how desperately America and the American people need change, and I know she will continue to be in the forefront of that battle this fall and for years to come."
His campaign website asked supporters to send a message of thank you to the Clinton campaign.
Earlier, Mrs Clinton formally suspended her 16-month-long campaign with a speech at the National Building Museum in Washington.
She opened by saying: "This isn't exactly the party I planned but I sure like the company."
Mrs Clinton thanked the "18 million of you from all walks of life" who voted for her and threw her support behind Mr Obama.
Although she had not succeeded, she said there were now "18 million cracks in the glass ceiling" preventing a woman from winning the White House.
Referring to her formal rival, she said: "I've had a front-row seat to his candidacy and I have seen his strength and determination, his grace and his grit."
Supporters of Mrs Clinton queued for up to six hours to hear her speech at the National Building Museum in Washington.
In an online message ahead of the speech, Mrs Clinton said she would focus on "how together we can rally the party" behind Mr Obama in the battle against Republican presumptive nominee, John McCain, in November's election.
"I have said throughout the campaign that I would strongly support Senator Obama if he were the Democratic party's nominee and I intend to deliver on that promise," she says.
But speculation will continue over whether Mrs Clinton will be offered the chance to run for vice president on the Democrat ticket, particulalry as she has only chosen to suspend her campaign rather than fully concede defeat.
She met with Mr Obama in Washington on Thursday evening, having a "productive session" according to a joint statement.
Mr Obama is believed to be concerned about the influence of former president Bill Clinton if he makes Mrs Clinton his running mate for the race for the White House.
Mrs Clinton has in the past said Bill and her "come as a package", a concept that might somewhat understandably cause Mr Obama a degree of discomfort.