WASHINGTON, DC - Exhorting her followers to "work as hard for Barack Obama as you have for me," Sen. Hillary Clinton today formally ended her historic, 16-month quest to become the first woman to win the White House, thanking her supporters and vowing to campaign for Sen. Barack Obama in his own groundbreaking bid to become the nation's first black president.

"This isn't the party I expected to have, but I sure like the company," she joked at the beginning of a 30-minute speech in which she congratulated her former opponent on "the victory he has won and the extraordinary race he has run. I endorse him, and throw my full support behind him."

"Today I am standing with Sen. Obama to say, 'yes we can,'" she said, citing Mr. Obama's trademark campaign mantra, to cheers -- and some scattered boos from diehard supporters who thronged the National Building Museum near the Washington Mall to hear her speak.

At times wistful, but mostly upbeat, she urged her followers to look forward.

"When you hear people saying,'what if, if only' please -- don't go there," she said.

"Every moment wasted looking back keeps us from moving forward. Life is too short, time is too precious and the stakes are too high to dwell on what might have been," she told the audience of more than a thousand. In this "turning point" of an election, she added, "it is critical that we all understand what our choice really is. Will we go forward together or will we stall and slip backwards?"

Standing under 89-foot columns in the Great Hall of the museum, which was built in the 19th century to provide pensions for civil war veterans, Mrs. Clinton was accompanied by her husband, former President Clinton, her mother, Dorothy Rodham, and daughter, Chelsea Cllinton.

Speaking clearly with an eye not just to her own future but to her husband's legacy, she pointed out that only three presidential elections out of 10 had been won by Democrats in the past 40 years, and "the man who won two of those elections is with us today," Clinton said.

To perhaps a greater extent than she had during the campaign, she talked about the role her gender played at a time when there are still "barriers and biases out there" against women.

"When we first started, people everywhere asked the same questions: Could a woman really serve as commander-in-chief? Well, I think we answered that one. And could an African American really be our President? Senator Obama has answered that one.

"From now on, it will be unremarkable for a woman to win primary state victories, unremarkable to have a woman in a close race to be our nominee, unremarkable to think that a woman can be the president of the

United States. And that is truly remarkable."

Among those cheering her on -- and still expressing bitterness at the closeness of the race and what they perceived as sexist treatment in the media -- was Darla Stone, 63, who said she and her husband left their home near Culpepper, Va. at dawn so they could get in line early and secure a front-row spot at the rally.

"I am an avid supporter of Hillary's" she said. "She'll probably get over this faster than I will."

Mrs. Stone, who works at Coffeewood Correctional Center in central Virginia, said she would rather vote for John McCain than Mr. Obama, who she called "an elitist. And I'm not alone. I work in a men's prison with blacks and Latinos who feel the same way I do."

Mrs. Stone expressed anger over what she felt were biased media portrayals of Mrs. Clinton -- along with sexism on the campaign trail, from shouts of "Iron my shirt," to the Hillary nutcracker and other gender-based insults.

"The media did her a great disservice and gave him a free ride," she said, adding that "they always published the ugliest photos of her. Why is that?"

She dismissed the notion of Mrs. Clinton as Mr. Obama's running mate. "It's a subordinate role. It's like being elected class secretary. I don't think I can stomach that," she said.

Sandy Ogas, of Annapolis, was there with her husband, John Ogas, an Obama supporter. "What makes me really sad is that I'll probably die without seeing a woman as president," she said.

"The country just didn't want a woman, and at the rate we're going it will be another 200 years before one of us gets in the White House," she said, while Mr. Ogas rolled his eyes.

"I don't argue with my wife in public, but the important thing is that Hillary Clinton is here today uniting the party behind Obama," he said.

On Tuesday, shortly after Mr. Obama won the 2,118 delegates he needed, Mrs. Clinton delivered a speech in which she refused to concede, saying she wanted time to talk to party leaders and supporters.

On Thursday she announced she was endorsing Mr. Obama and suspending her campaign -- not ending it, so that under party rules delegates still being slated at various state conventions will be allowed to go to the party's national convention in August.

Mrs. Clinton's speech today came two days after she met secretly with Mr. Obama for an hour Thursday night at the Washington home of Sen. Dianne Feinstein, D-California, an early supporter of Mrs. Clinton. Afterward they issued a joint statement saying they had discussed the campaign, but other details weren't forthcoming. Mrs. Clinton has sought to dampen speculation in recent days that she was angling to be Mr. Obama's running mate, but she will seek his help in retiring her $30 million debt and vowed to campaign strenuously for him.

Mrs. Clinton's journey from front-runner to also-ran began in January of 2007, when she announced in a video posted on her Web site that she was "in it to win it," and indeed, she was leading in the polls, fundraising and endorsement. But a few dramatic early victories never materialized: instead, she stumbled after a third-place loss in the Iowa caucuses early this year. While she recovered with a surprise win in New Hampshire, Mr. Obama's wins in 11 straight primaries gave him a delegate lead she could never surmount.