WASHINGTON — Senator Clinton ended her historic campaign for the presidency today and told supporters to unite behind Senator Obama, closing out a race that was as grueling as it was groundbreaking.

Mrs. Clinton, who as recently as Tuesday declared herself the strongest candidate, gave Mr. Obama an unqualified endorsement and pivoted from her role as determined foe to absolute ally.

“The way to continue our fight now to accomplish the goals for which we stand is to take our energy, our passion, our strength and do all we can to help elect Barack Obama, the next president of the United States,” she said in a speech before cheering supporters packed into the ornate National Building Museum, not far from the White House she longed to govern from.

“Today as I suspend my campaign, I congratulate him on the victory he has won and the extraordinary race he has run. I endorse him and throw my full support behind him and I ask of you to join me in working as hard for Barack Obama as you have for me,” Mrs. Clinton said in her 28-minute address.

With that and 13 other mentions of his name, Mrs. Clinton placed herself solidly behind her Senate colleague from Illinois, a political sensation and the first black to secure a presidential nomination.

For Mrs. Clinton and her supporters, it was a poignant moment, the end of an extraordinary run that began with an air of inevitability and certain victory. About 18 million people voted for her; it was the closest a woman has come to capturing a nomination.

“Although we weren’t able to shatter that highest, hardest glass ceiling this time, thanks to you, it has about 18 million cracks in it and the light is shining through like never before,” she said.

Indeed, her speech repeatedly returned to the milestone her candidacy represented for women. In primary after primary, her support among women was a solid bloc of her voting coalition. She noted that she’d received the support of women who were 80 and 90 years old, born before women could even vote.

She acknowledged the unprecedented success of Mr. Obama’s candidacy, as well.

“Children today will grow up taking for granted that an African-American or a woman can, yes, become the president of the United States,” she said.

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

Joining Mrs. Clinton on stage were President Clinton and their daughter, Chelsea, to loud cheers from the crowd. When she spoke, they stepped away.

In deciding to suspend her campaign, Mrs. Clinton kept some options open. She gets to retain her delegates to the nominating convention this summer and she can continue to raise money. It also means she could reopen her campaign if circumstances change before the Denver convention, but gave no indication that was her intention.

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

Supporters and press jammed the museum’s vast ground floor, with the second and third floor balconies quickly filling up as well. The stage was draped with American flags, and a sound system blared upbeat music.

As they awaited her arrival, campaign staffers milled the room, exchanging hugs and saying goodbye.

Mrs. Clinton seemed almost buoyant in her address, feeding off the energy of a loud and appreciative crowd.

“Well, this isn’t exactly the party I planned but I sure like the company,” she said as she opened her speech.

Mrs. Clinton’s backers described themselves as sad and resigned. “This is a somber day,” said Jon Cardinal, one of the first in line. Mr. 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.

Mr. Obama and Mrs. Clinton had a face-to-face meeting Thursday evening at the Washington home of a Senate colleague, Dianne Feinstein.

Mrs. Clinton was expected to campaign for Mr. 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 Mr. Obama as his running mate.

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