Hillary Clinton has formally suspended her campaign to become the Democratic nominee for US president.
She said she was "grateful to everyone who poured [their] hearts and hopes into this campaign" and threw all her support behind Barack Obama.
Mrs Clinton was given a huge ovation for her speech in Washington.
She thanked the "18m of you from all walks of life who voted" for her.
Mrs Clinton said the Democratic party was "a family and it's time to restore the ties that bind us together".
She said: "I intended to win back the presidency... and that's what we must do by ensuring Barack Obama walks through the doors of the Oval Office in January."
She added: "You'll always find me on the front lines of democracy fighting for the future."
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.
The New York senator had clung on against rival Barack Obama since losing the opening primary in Iowa in January, always bouncing back in must-win races.
But with the final primaries wrapped up on Tuesday, Mr Obama emerged as the man with the most delegates. He will now face the Republican nominee John McCain in the race for the White House.
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.