Democratic U.S. presidential hopeful Barack Obama, the first African American to be a major party nominee for the White House, waves to a crowd in St Paul, Minn., on Tuesday. (Chris Carlson/Associated Press)Hillary Clinton will end her presidential campaign and express support for rival Barack Obama on Saturday, a Clinton campaign official said Wednesday, drawing a lengthy and often rancorous battle for the U.S. Democratic presidential nomination to a close.

Obama, an Illinois senator, garnered enough delegate support Tuesday to clinch the nomination at the party's national convention in August, but Clinton refused to concede on the night of his victory.

On Wednesday, hours after U.S. media reports said Clinton would drop out of the race at the end of the week, her communications director, Howard Wolfson, said in a statement that the New York senator and former first lady will be hosting an event in Washington, D.C., on Saturday "to thank her supporters and express her support for Senator Obama and party unity."

An earlier statement from the campaign said the event was to be held on Friday.

"This event will be held on Saturday to accommodate more of Senator Clinton's supporters who want to attend," Wolfson said, while adding Clinton was leaving her options open to retain her delegates and promote her issue agenda.

Universal health care, Clinton's signature issue as the president's wife in the 1990s, was a point of dispute between Obama and her during their epic nomination fight.

Also in the speech, Clinton will urge once-warring Democrats to focus on the general election and defeating presumptive Republican presidential nominee John McCain.

Once favoured, defiant to the last vote

The announcement capped the long and turbulent campaign of Clinton, who entered the race 17 months ago heavily favoured in a crowded Democratic field, with a fundraising machine and battle-hardened, loyal campaign team already assembled.

But the message of change from the lesser-known, yet charismatic Obama appeared to spark a chord with Democratic voters, generating a surge in Democratic membership and a leadoff victory in the Iowa caucuses in January.

Written off ahead of New Hampshire's primary, Clinton surprised many by winning the state, with observers crediting a rare display of emotion as resonating with the state's voters.

Then came South Carolina and perhaps the greatest blunder committed by her most vocal supporter and husband, former president Bill Clinton, who sparked controversy by invoking Jesse Jackson's primary victories in the state in previous Democratic nomination races.

Many African-American supporters who had supported Bill Clinton in his successful presidential runs were outraged, believing he was suggesting Obama's victory in the state was merely because of his race, a charge the former president furiously denied.

Then, after a roughly even finish on Super Tuesday Feb. 5, Hillary Clinton suffered a string of unanswered losses that, almost before she noticed, put Obama so far ahead in the delegate hunt that all the big-state victories she piled up couldn't close the delegate gap.

By March, her options limited, Clinton adopted the persona of a tenacious fighter for the middle class as America's economy appeared set to falter further. As her rival struggled to contend with controversial comments made by his outspoken former pastor, the Rev. Jeremiah Wright, Clinton powered successfully through primaries in states like Ohio, Pennsylvania, Indiana, West Virginia and Kentucky, showing grit that earned her valuable political currency.

On Tuesday, as Obama claimed victory in the overall delegate race, a defiant Clinton would not end her campaign, saying she would consult with supporters and party leaders before making a decision.

Obama launches VP search committee

The announcement by the Clinton camp came after Obama formed a high-profile committee to help search for a running mate. Clinton has indicated interest in the vice-presidential slot.

Obama's selection committee comprises Caroline Kennedy, daughter of former U.S. president John F. Kennedy; former U.S. deputy attorney general Eric Holder; and Fannie Mae CEO Jim Johnson, who played the same role for 2004 Democratic presidential nominee John Kerry.

"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."

Democratic presidential hopeful Hillary Clinton speaks to a large crowd of supporters on Tuesday night in New York City. (Julie Jacobson/Associated Press) Top Democratic party officials have called for unity behind Obama. Dozens of superdelegates have yet to announce whom they support and the party leadership called for them to do so by Friday.

"Democrats must now turn our attention to the general election," said the statement, signed by the party's leaders in Congress, the head of the Democratic state governors' association and chair of the Democratic National Committee.

The call came as Obama supporters criticized Clinton for not formally withdrawing from the race for the party's nomination Tuesday night.

Obama, who spoke Wednesday to CNN on Capitol Hill, said he'd had a conversation with Clinton earlier in the day. Both were in Washington to deliver speeches.

"I'm very confident of how we're going to be able to bring the party together," said Obama, who said he planned to speak to the New York senator again in the coming weeks.

He said he was honoured and humbled to be the presumptive nominee.

"You think about all the people who had to knock down barriers for me to walk through this door," he told CNN. "And the challenges they went through were so much more difficult, so much more severe, and the risks they took were so much greater that I will say, last night standing in that auditorium, it struck me that it was testimony to them."

Fighting McCain should be 'job one'

CBC's Henry Champ in Washington said there's real concern among Democrats that the party's campaign for the White House might be damaged by Clinton's refusal to step down and endorse Obama immediately after his victory.

"It has become job one for the Obama campaign and the Democratic party to end this issue of Hillary," Champ says. "Many people were hoping that job one today would be taking on John McCain and getting down to the business of fighting Republicans.

"It’s going to be a number of days before this works out."

For his part, Obama praised Clinton in his speech Wednesday to the main lobby group for Israel in the United States, the American Israeli Public Affairs Committee.

Clinton, he said, had run "an outstanding campaign and made history alongside me for the past 18 months." But he gave no indication of what he planned to do to win her public endorsement for his presidential candidacy.

In a separate appearance before the group later Wednesday, Clinton said, "It has been an honour to contest with Senator Obama, an honour to call him my friend and I know that Senator Obama will be a good friend to Israel."

She said the U.S. needs a Democrat in the White House after the November election but gave no explicit indication that she would throw her support behind Obama.

Supporters demand Clinton be VP candidate

Obama's advisers have acknowledged that Clinton has expressed interest in being Obama's vice-presidential nominee, but no decision has been made, they said.

"I think a lot of her supporters would like to see her on the ticket," Clinton campaign chair Terry McAuliffe said Wednesday. But Obama spokesman Robert Gibbs cautioned "there is no deal in the works."

In his St. Paul speech, Obama also turned his attention to the presumptive Republican party nominee, McCain, accusing him of being too close to the unpopular Republican President George W. Bush.

"While John McCain can legitimately tout moments of independence from his party in the past, such independence has not been the hallmark of his presidential campaign," he said.

In what is bound to become a signature theme of his Republican push for the White House, McCain emphasized his record and experience in public life.

"I have a few years on my opponent," the 71-year-old Arizona senator said in New Orleans on Tuesday, "so I'm surprised that a young man has bought into so many failed ideas."

Obama, 46, is a first-term senator.

With files from the Associated Press