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

Hours after Obama sealed the nomination, Democrats coalesced around his candidacy, sending a signal to Clinton that it was time to bow out. The former first lady told House Democrats during a private conference call yesterday 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," said Howard Wolfson, her communications director.

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

The announcement brought to a close a five-month nominating battle pitting a 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.

Yesterday, Clinton's supporters were urging Obama to put her on the ticket now that she's decided to back his inevitable nomination. Obama indicated he would at least consider his rival for the job of vice president.

"Senator Clinton would be on anybody's short list, obviously," he told CBS News, adding they agree on most issues.

He told ABC News that Clinton is a "special case" to consider as a candidate for the vice presidency.

"She's somebody who traveled this journey with me. She was extraordinarily capable and tenacious. I mean, she is just a great candidate," Obama said.

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

Other options included freeing her delegates to back Obama and ending her candidacy unconditionally. The official stressed that neither Clinton nor her inner circle had decided 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.

SUPPORTERS IMPATIENT

In the telephone call with impatient congressional supporters including New York Rep. Charles Rangel, a longtime political patron, Clinton was urged to end the contentious campaign, or at least express support for Obama.

Rangel said it was "confusing" that she hadn't endorsed Obama. He said he would back her efforts to join Obama as his vice presidential nominee. But, he added, it is incumbent on her to acknowledge she has lost the fight to Obama.

"What I don't know is what the heck she needs this extra time for," he said, referring to Clinton's speech Tuesday, in which she said she would take a few days to consider her options. "How much more time does she need to be able to say the person she wants to help is Barack Obama? I don't know what this intrigue is all about."

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.

Clinton visited her campaign's northern Virginia headquarters after speaking to the American Israel Political Affairs Committee and told staffers that they would be paid through June 15 but that tomorrow would be their last day of work.

'SURE THING' FIZZLES OUT

Obama and Clinton gave back-to-back speeches at the AIPAC conference, expressing their commitment to the security of Israel. Clinton used the occasion to give some acknowledgement that her struggle for the nomination was over.

Speaking of the need for a "Democratic president," she assured the audience, "I know, I know Sen. Obama understands what is at stake here. It has been an honor to contest these primaries with him. It is an honor to call him my friend. And let me be very clear, I know Sen. Obama will be a good friend to Israel."

With Clinton 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 her victory in the Democratic nomination contest was a sure thing when it began 17 months ago.

But 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: She was up in Nevada, down in South Carolina. Then, after a roughly even finish on Super Tuesday, she suffered a string of losses that put Obama so far ahead in the delegate hunt that all the big-state victories she piled up couldn't close the gap.

USA Today and the Washington Post contributed to this report.