WASHINGTON - Her path to the nomination inevitable no more, Hillary Rodham Clinton plans to announce she is ending her groundbreaking candidacy and supporting Barack Obama, her rival in a presidential quest for the ages.

Clinton prepared to declare Saturday that she is backing the Illinois senator after Democratic congressional colleagues made clear they had no stomach for a protracted intraparty battle once Obama secured the 2,118 delegates necessary to clinch the nomination.

A presidential contender who announced 17 months ago that she was "in it to win it," the former first lady plans to end her quest with a more humble plea for party unity.

In truth, she had little choice.

Hours after Obama sealed the nomination, Democrats coalesced around his candidacy, sending a strong signal to Clinton that it was time to bow out. The New York senator told House Democrats during a private conference call Wednesday that she would express support for Obama's candidacy and congratulate him for gathering the necessary delegates to be the party's nominee.

"Senator Clinton will be hosting an event in Washington, D.C., to thank her supporters and express her support for Senator Obama and party unity. This event will be held on Saturday to accommodate more of Senator Clinton's supporters who want to attend," her communications director, Howard Wolfson, said.

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

The only degree of uncertainty was how. Clinton is exploring options to retain her delegates and promote her issues, including a signature call for universal health care.

The announcement closed an epic five-month nominating battle pitting the first serious female candidate against the most viable black contender ever.

Obama on Tuesday night secured the delegates needed to clinch the Democratic nomination. But Clinton stopped short of acknowledging that milestone, defiantly insisting she was better positioned to defeat McCain in November.

"What does Hillary want? What does she want?" Clinton asked, hours after telling supporters she'd be open to joining Obama as his vice presidential running mate.

But by Wednesday, other Democrats made it abundantly clear they wanted something too: a swift end to the often bitter nominating contest.

Democratic Party Chairman Howard Dean and the Democratic congressional leadership released a statement urging the party to rally behind Obama, and several lawmakers, including Iowa Sen. Tom Harkin, Colorado Sen. Ken Salazar and Louisiana Sen. Mary Landrieu, all endorsed their Illinois colleague.

Obama also announced he had named a three-person vice presidential vetting team that included Caroline Kennedy, daughter of the late President Kennedy.

On the telephone call with impatient congressional supporters that included New York Rep. Charles Rangel, a longtime political patron, Clinton was urged to draw a close to the contentious campaign, or at least express support for Obama. Her decision to acquiesce caught many in the campaign by surprise and left the campaign scrambling to finalize the logistics and specifics behind her campaign departure.

It was an inauspicious end for a candidacy that appeared all but indestructible when it began Jan. 20, 2007.

Armed with celebrity, a prodigious fundraising network, a battle-tested campaign team and husband who also was a popular two-term former president, Clinton was believed by many observers to be unbeatable.

But in Obama, the New York senator faced an opponent who appeared perfectly suited to the time , a charismatic newcomer who had opposed the Iraq war from the beginning , in contrast to her , and who offered voters a compelling message of change. Clinton voted for the legislation that authorized military force against Iraq, a decision that hampered her campaign from the beginning.

After a disastrous showing in the leadoff Iowa caucuses on Jan. 3, Clinton won New Hampshire's primary Jan. 8, setting off the state-by-state war of attrition with Obama that followed.

Her fortunes rose and fell like a fever chart: She was up in Nevada, down in South Carolina. Then, after a roughly even finish on Super Tuesday Feb. 5, she suffered a string of unanswered losses that, almost before Clinton 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. She powered successfully through primaries in states like Ohio, Pennsylvania, Indiana, West Virginia and Kentucky, showing grit that earned her valuable political currency.

White men, blue-collar workers, socially conservative Democrats and older women were especially receptive to her message, and her strong showing with those voters exposed Obama's vulnerabilities among those groups.

Democrats whose No. 1 concern had been ending the Iraq war at the campaign's outset started worrying more about the economy. That was a switch from Obama's strength to hers.