Hillary Clinton will declare her strong support for Barack Obama's White House bid and rally supporters around him, she said in a letter today, drawing the curtain on a gruelling 16-month nomination fight that badly split the Democratic Party.
Mrs Clinton will publicly back Mr Obama on Saturday and pledge to work for party unity in the general-election race against Republican John McCain.
"On Saturday, I will extend my congratulations to Senator Obama and my support for his candidacy," the New York senator and former first lady said in a letter to her backers released early this morning.
Mrs Clinton has not decided whether to officially close the campaign
"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."
Mrs Clinton confirmed she would hold an event in Washington on Saturday to thank everyone who had backed her campaign. The event was originally planned for tomorrow but the day was switched to allow more supporters to attend.
"This has been a long and hard-fought campaign, but as I have always said, my differences with Senator Obama are small compared to the differences we have with Senator McCain and the Republicans," she said in the letter.
"I will be speaking on Saturday about how together we can rally the party behind Senator Obama. The stakes are too high and the task before us too important to do otherwise."
Mrs Clinton has not decided whether to officially close the campaign or suspend it, allowing her to keep control of her delegates to the nominating convention, aides said.
She spent much of yesterday talking to supporters, many of whom urged her to halt her bid now that Mr Obama has clinched the nomination. Mr Obama attended two fund-raising events in New York City last night and acknowledged her decision.
"Your junior senator from New York engaged in an extraordinary campaign," he told attendees at one fund-raiser.
"Now that the interfamily squabble is done, all of us can focus on what needs to be done in November."
Mr Obama, the first black candidate to lead a major US party into a White House race, announced a three-member team to head his search for a running mate as he began the task of unifying the party the day after clinching the nomination.
Mr McCain proposed that Mr Obama join him for a series of joint summer town-hall meetings across the country. Mr Obama's campaign manager called the idea "appealing" but proposed format changes and made no immediate commitment.
Caroline Kennedy, daughter of the late President John F Kennedy, will vet prospective Obama running mates along with former Deputy Attorney General Eric Holder and Jim Johnson, former chief executive of the mortgage lender Fannie Mae, who performed the same task for Democrats John Kerry in 2004 and Walter Mondale in 1984.
Near the top of their agenda will be questions about a possible teaming with Mrs Clinton, who has indicated interest in the job after her presidential bid fell short.