Senator Hillary Rodham Clinton will endorse Senator Barack Obama on Saturday, 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 Obama.

Howard Wolfson, one of Clinton’s chief strategists, and other aides said she would express support for Obama and party unity at an event in Washington that day. One adviser said Clinton would concede defeat, congratulate Obama and proclaim him the party’s nominee, while pledging to do what was needed to assure his victory in November.

Her decision came after a day of conversations with supporters on Capitol Hill about her future now that Obama had clinched the nomination. Clinton had, in a speech after Tuesday night’s primaries, suggested she wanted to wait before deciding about her future, but in conversations 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 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.”

Clinton’s decision came as some of her most prominent supporters — including former Vice President Walter F. Mondale — announced they were now backing Obama. “I was for Hillary — I wasn’t against Obama, who I think is very talented,” Mondale said. “I’m glad we made a decision and I hope we can unite our party and move forward.”

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