WASHINGTON – Hillary Rodham Clinton is ending her historic bid to become the first female president and will back rival Barack Obama on Saturday, capping a 17-month quest that began with the words ”I’m in it to win it” with a more humble plea for party unity.

Hours after Barack Obama sealed the nomination, Democrats coalesced around his candidacy, sending a strong signal to Clinton that it was time to bow out. The former first lady told House Democrats during a private conference call Wednesday that she will 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 Tuesday night secured the 2,118 delegates to claim the Democratic nomination, but Clinton stopped short of acknowledging that milestone. Instead, she was defiant, insisting she was better positioned than Obama to defeat McCain in November.

”What does Hillary want? What does she want?” Clinton said, 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 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 vetting team that included Caroline Kennedy, daughter of the late President John F. Kennedy.

An adviser said Clinton and her lieutenants had discussed various ways a presidential candidacy can end, including suspending the campaign to retain control of her convention delegates and sustain her visibility in an effort to promote her signature issue of health care. This adviser spoke on condition of anonymity because officials were not authorized to discuss the conference call Clinton held with her congressional supporters.

Other options include freeing her delegates to back Obama and ending her candidacy unconditionally. The official stressed that neither Clinton nor her inner circle had decided specifically what course to take other than to recognize that the active state of her bid to become the nation’s first female president had ended.

On the telephone call with impatient congressional supporters including 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 indestructible when it began 17 months ago.

Armed with celebrity, a prodigious fundraising Rolodex, a battle-tested campaign team and a popular two-term former president as a husband, many observers believed Clinton’s victory in the Democratic nomination contest was a sure thing.

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

After a disastrous showing in the leadoff Iowa caucuses 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, and 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.