Senator Hillary Rodham Clinton will endorse Senator Barack Obama on Friday, bringing a close to her 17-month campaign for the White House, aides said. Her decision came after Democrats urged her on Wednesday to leave the race and allow the party to coalesce around Mr. Obama.

    Mrs. Clinton's aides said she would "express her support for Senator Obama and party unity" at an event in Washington that day. One adviser said that Mrs. Clinton would concede defeat, congratulate Mr. Obama and proclaim him the party's nominee, while pledging to do what was needed to assure his victory.

    Her decision came after a day of conversations with supporters on Capitol Hill about her future now that that Mr. Obama had clinched the nomination. Mrs. Clinton had, in a speech after Tuesday night's primaries, suggested she wanted to wait before deciding about her future, but in conversations throughout the day on Wednesday, her aides said, she was urged to step aside.

    "We pledged to support her to the end," Representative Charles B. Rangel, a New York Democrat who has been a patron of Mrs. Clinton since she first ran for the Senate, said in an interview "Our problem is not being able to determine when the hell the end is."

    Mrs. Clinton's decision came as some of her most prominent supporters - including former Vice President Walter F. Mondale - announced that they were now backing Mr. Obama.

    "I was for Hillary - I wasn't against Obama, who I think is very talented," Mr. Mondale said. "I'm glad we made a decision and I hope we can unite our party and move forward."

    One of Mrs. Clinton's aides said they were told that except for her senior advisers, there was no reason to report to work after Friday, and that they were invited to Mrs. Clinton's house for a farewell celebration that afternoon.

    "Senator Clinton will be hosting an event in Washington, D.C., on Friday to thank her supporters and express support for Senator Obama and party unity," said Howard Wolfson, one of her chief strategists.

    Earlier Wednesday, a group of top Democratic leaders asked all of the party's uncommitted superdelegates, the officials and party leaders who get automatic convention seats, to make their preferences known by Friday.

    While the group of leaders - including the chairman of the Democratic National Committee, Howard Dean; the speaker of the House, Nancy Pelosi; the Senate majority leader, Harry Reid; and Gov. Joe Manchin of West Virginia - did not formally endorse Mr. Obama or urge Mrs. Clinton to exit the race, they said in a joint statement: "Democrats must now turn our full attention to the general election."

    Facing a fall campaign against Senator John McCain, the presumptive Republican nominee, these Democratic leaders stressed that the party needed to "stand united and begin our march toward reversing the eight years of failed Bush/McCain policies that have weakened our country."

    Other party leaders began to coalesce behind Mr. Obama, including Representative Rahm Emanuel, of Illinois, a former aide in the Clinton White House but also a close friend of Mr. Obama.

    "Look, I've known him for years, and I said that as the election came to an end, I would make my endorsement, come from underneath the desk - and I did that," Mr. Emanuel said. "The fact is, he is the nominee."

    Some of Mrs. Clinton's supporters urged Mr. Obama to take the senator on as his running mate.

    Robert L. Johnson, a prominent Clinton backer and the founder of Black Entertainment Television, said Wednesday on the CNN program "American Morning" that he planned to enlist members of the Congressional Black Caucus to push Mr. Obama to accept Mrs. Clinton as his vice presidential nominee. He said Mrs. Clinton had not directed his efforts, but was aware of them.

    Lanny Davis, who was an aide in the Clinton White House, said he was circulating a petition asking Mr. Obama to pick Mrs. Clinton as his running mate. Mr. Davis said he was acting on his own.

    On a conference call with members of the New York Congressional delegation on Tuesday, Mrs. Clinton was asked whether she would be open to joining a ticket headed by Mr. Obama. She replied, according to some who were on the call, that if he offered the vice presidential nod, she would accept, and would do whatever she could to help Democrats win the White House.

    Terry McAuliffe, the Clinton campaign chairman, however, insisted Wednesday on CNN that there had been "absolutely zero discussions" about whether she would accept a vice presidential nomination. Mr. McAuliffe said that Mrs. Clinton, whose speech on Tuesday night in New York was more defiant than conciliatory, wanted to talk things over with her supporters on Wednesday.

    Mr. Obama and Mrs. Clinton spoke by telephone just after midnight on Wednesday. He congratulated her and renewed his offer to "sit down when it makes sense for you," according to a spokesman for Mr. Obama, Robert Gibbs. Mrs. Clinton responded positively, Mr. Gibbs said, but he added there were no immediate plans to meet.

    On Wednesday, Mr. Obama was received like a hero when he showed up on the Senate floor around midday to vote on the final version of a budget blueprint for 2009. Senators from both parties embraced him and congratulated him on having clinched the Democratic presidential nomination.

    Mr. Obama hugged two of his early supporters, Senators Kent Conrad of North Dakota and Claire McCaskill of Missouri. He kissed Senator Blanche Lincoln of Arkansas, who had endorsed Mrs. Clinton. He shook hands with Senators Barbara A. Mikulski of Maryland, a Clinton supporter, and Saxby Chambliss of Georgia, a Republican.

    Mr. Obama huddled for several minutes of intense conversation in a corner of the Senate chamber with Senator Joseph I. Lieberman of Connecticut, the onetime Democrat who became an independent and has endorsed Mr. McCain.

    Earlier in the day, both Mr. Obama and Mrs. Clinton addressed the American Israel Public Affairs Committee, the most prominent pro-Israel lobbying group in Washington. Mr. Obama took the stage first, and immediately offered warm words for Mrs. Clinton, saying, "She has made history alongside me over the last 16 months."

    A short while later, Mrs. Clinton came to the podium and plunged straight into her speech, ignoring speculation about her political intentions. She digressed, however, to offer this assurance to her audience: "I know Senator Obama will be a good friend to Israel."

    Mr. Obama has struggled to combat the wariness about him that has been harbored by some Jewish voters, a wariness evidently fueled by e-mail messages spreading false rumors about his background and positions. In his speech, he promised to be uncompromising in his defense of Israel's security. He also revived the contentious issue of how much to engage foreign adversaries, especially Iran, which promises to be a central dispute in his general election battle with Mr. McCain.

    "Contrary to the claims of some, I have no interest in sitting down with our adversaries just for the sake of talking," Mr. Obama said. "But as president of the United States, I would be willing to lead tough and principled diplomacy with the appropriate Iranian leader at a time and place of my choosing - if, and only if, it can advance the interests of the United States."

    Mr. McCain has attacked Mr. Obama for his response at a Democratic debate last year, in which he said he would be willing to sit down with the leaders of some America's most ardent foes, including Iran, "without preconditions."

    But Mr. Obama said that a change in course was needed in the country's diplomatic approach, and that Mr. McCain "refuses to understand or acknowledge the failure of the policy he would continue."

    For her part, Mrs. Clinton seemed to take pains to avoid bringing up issues where she and Mr. Obama have differed, including the question of when to meet with certain adversaries; during the primary campaign she, too, criticized Mr. Obama's stance.

    It was a last-minute rush of commitments by Democratic superdelegates, as well as the results from the final two primaries in Montana and South Dakota on Tuesday, that raised Mr. Obama's total above the 2,118-delegate threshold needed to secure the nomination at the party's convention in August. The victory for Mr. Obama, the son of a black Kenyan father and a white Kansan mother, broke racial barriers and represented a remarkable rise for a man who just four years ago was serving in the Illinois State Senate.

    But to the end, the primaries reflected the party's continuing divisions, as Mrs. Clinton won the South Dakota contest while Mr. Obama won in Montana.

    The competition between Mr. McCain and Mr. Obama has been sharpening for weeks, but the close of the Democratic primary race formally raised the curtain on a five-month general election contest.

    At a town-hall-style meeting in Baton Rouge on Wednesday, Mr. McCain announced that he had invited Mr. Obama to join him in a series of 10 such meetings between now and the Democratic convention in late August.

    "I think Americans are tired about the ways presidential campaigns have been run in the past - all the gimmicks, the phony sound bits and photo-ops," Mr. McCain said. "Campaigns always seemed to be more about the candidates' interest than the public."

    Mr. McCain even proposed a date for the first event: June 12 in New York City.

    Mr. Obama had earlier indicated that he might be open to such a proposal, though it is clearly to the McCain campaign's advantage to push the concept. Mr. McCain will almost certainly be widely outspent by Mr. Obama, whose fund-raising has far outstripped his opponent's, and joint appearances where the candidates would get equal attention could help offset the disparity.

    Mr. McCain even joked about the financial aspect, suggesting that the candidates share the same airplane to the forums: "Given our expenses," he said, "I know my campaign would agree to it."

    David Plouffe, the Obama campaign manager, later issued a statement saying the town-hall proposal was "appealing, and one that would allow a great conversation to take place about the need to change the direction of this country."

    But he said that his side would recommend a "less structured and lengthier" format, "one that more closely resembles the historic debates between Abraham Lincoln and Stephen Douglas."

    Nevertheless, Mr. Plouffe said he looked forward to continuing the discussion over the matter with the McCain campaign.

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    Jeff Zeleny, Carl Hulse, David E. Sanger and Robert Pear contributed reporting.