Hillary Clinton will declare her strong support for Barack Obama's White House bid and rally supporters around him, she said in a letter on Thursday, drawing the curtain on a gruelling 16-month nominating fight that badly split the Democratic Party.

Clinton will publicly back 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 on Thursday morning.

"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."

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 Friday 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."

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.

Clinton spent much of Wednesday talking to supporters, many of whom urged her to halt her bid now that Obama has clinched the nomination. Obama attended two fund-raising events in New York City on Wednesday 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."

RUNNING MATE

Obama, the first black candidate to lead a major U.S. 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.

McCain proposed that Obama join him for a series of joint summer town-hall meetings across the country. 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 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 Clinton, who has indicated interest in the job after her presidential bid fell short.

"We're going to be having a conversation in coming weeks," Obama told reporters when asked about the former first lady. He said he was confident the party would be unified to win the general election.

After news of Clinton's decision broke, Obama adviser Linda Douglass told reporters she ran a great race.

"Her supporters have every reason to gather and celebrate that and we're confident the party will be united with her help," she said.

Clinton's supporters turned up the pressure for her to be named as Obama's vice presidential candidate. Robert Johnson, founder of Black Entertainment Television, said he wrote to the Congressional Black Caucus urging members to push Obama to choose Clinton.

Obama's campaign said the search was just beginning.

"Senator Obama is pleased to have three talented and dedicated individuals managing this rigorous process," spokesman Bill Burton said. "He will work closely with them in the coming weeks but ultimately this will be his decision and his alone."

The victory by Obama, son of a black Kenyan father and white mother from Kansas, marked a milestone in U.S. history. It came 45 years after the height of the civil rights movement and followed one of the closest and longest nomination fights in recent U.S. political history.