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 Mr. Obama.

Howard Wolfson, one of Mrs. Clinton’s chief strategists, and other aides said she would express support for Mr. 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 Mr. Obama had clinched the nomination. Mrs. Clinton had, in a speech after Tuesday night’s primaries, suggested that 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,” said Representative Charles B. Rangel, a New York Democrat who has been a patron of Mrs. Clinton since she first ran for the Senate. “Our problem is not being able to determine when the hell the end is.”