WASHINGTON—Hillary Rodham Clinton has ended her historic quest for the presidency and endorsed Democratic rival Barack Obama.

AP

Sen. Hillary Rodham Clinton, D-NY, greets supporters at the National Building Museum in Washington, Saturday as she suspends her campaign for president. (AP)

The former first lady told supporters packed into the ornate National Building Museum on Saturday that she was withdrawing from the race.

She is urging her backers to take all their energy and passion and do everything they can to help elect Obama the next president.

Obama secured the 2,118 delegates needed to clinch the Democratic presidential nomination Tuesday after primaries in South Dakota and Montana. He planned to spend the weekend at home in Chicago.

Supporters began lining up at dawn to attend the farewell address. A smattering of Obama backers showed up as well, saying they did so as a gesture of party unity.

“We’re all Democrats,” said Andy Cunningham, a 22-year-old Obama supporter. “She’s contributed a lot to the party.”

Clinton backers described themselves as sad and resigned. “This is a somber day,” said Jon Cardinal, one of the first in line.  Cardinal said he planned, reluctantly, to support the Illinois senator in the general election. “It’s going to be tough after being against Obama for so long,” he said.

Aides said Clinton would be unequivocal in her praise for Obama, her rival in an epic, 50-state nominating contest pitting the first serious female candidate for president against the most viable black candidate.

“I have said throughout the campaign that I would strongly support Senator Obama if he were the Democratic Party’s nominee and I intend to deliver on that promise,” Clinton told supporters in an online message late this week.

She said her speech would focus on “how together we can rally the party” behind Obama as he prepares for the general election against Arizona Sen. John McCain. “The stakes are too high and the task before us too important to do otherwise.”

The two had a face-to-face meeting Thursday evening at the Washington home of a Senate colleague, California Democrat Dianne Feinstein, where they discussed the campaign to come. They spoke alone for about an hour. Both were laughing when they finished.

Clinton was expected to campaign for Obama and to help with fundraising, while seeking his assistance in retiring her $30 million campaign debt.

The New York senator has told colleagues she would be interested in joining Obama as his running mate.

Clinton spent much of Friday working on her concession speech with campaign manager Maggie Williams, media adviser Mandy Grunwald and strategist Mark Penn.

A party at her Washington home on Friday was intended as a way to thank and bid farewell to campaign staff.

Clinton and Obama went to great lengths to keep their meeting a secret from the media beforehand.

Obama “was very gracious,” said Feinstein, who had supported Clinton during the primaries. “He said he would go wherever, whenever Senator Clinton wanted.”

“This is a deeply personal time, too, you know,” Feinstein said. “Barack is trying to put things together for a major presidential campaign. There are a lot of decompression and nerve endings that need to come together.”

The undisputed front-runner when she announced her candidacy in January 2007, Clinton saw her march to the nomination derailed a year later after being swamped by Obama in Iowa’s leadoff caucuses.  She stayed alive after a narrow victory in New Hampshire five days later. But her campaign never fully regained its footing despite strong showings in several big-state primaries beginning in March.