WASHINGTON - Waving from his wheelchair as if in a convertible at a parade, Strom Thurmond, the longest-serving and oldest senator ever, celebrated his 100th birthday at a Capitol Hill bash yesterday, topping off a career as remarkable for its length as for its role in political history.

Hundreds of friends, relatives and staff aides past and present made their way to a Capitol Hill blanketed with snow to celebrate at a private party in the Senate's Dirksen office building.

The frail centenarian - whose transformation from a leading segregationist Democrat to conservative Republican tracked a broader shift in the South and the nation - was given a giant American-flag cake bearing 100 red, white and blue candles.

In a spoof of a famous John F. Kennedy episode, Thurmond, known as a ladies' man, was treated to a Marilyn Monroe impersonator who sang a sultry "Happy Birthday, Mr. President Pro Tempore." (The title refers to an honor traditionally bestowed on the senior senator of the majority party.)

Hundreds of South Carolinians and several prominent Washington figures, including Bob Dole, a former Republican Senate leader, and Trent Lott, the party's leader in the Senate, turned out, as did several Supreme Court justices.

"Strom's life tracks the trajectory of the 20th century," said Lott, who called him "a towering figure in the history of the Senate."

Though he never shepherded a major piece of legislation during his 48-year career, Thurmond enjoys the renown that comes with having outlived and outserved everyone in a chamber that so values seniority.

"He is considered an institution within the institution and is widely revered for that," said Thad Strom, a distant cousin who served as an aide for 25 years and helped plan yesterday's party.

Thurmond has long since ceased to be a force on Capitol Hill. As he has weakened with age, Thurmond has seemed to struggle to perform the mundane tasks of a senator, such as attending committee meetings and voting on the Senate floor.

It is widely known that a handful of top aides have run his office and effectively handled his senatorial duties.

But many of his colleagues, even those who disagree with his politics, have regarded him as a historical treasure. His life and career have tracked many political milestones of this century, especially in the South.

Born in 1902 in Edgefield, S.C., during the first administration of Theodore Roosevelt, Thurmond won his first elected office as a local education official at age 26.

In 1948, he ran for president as one of the "Dixiecrats" - a group of conservative Democrats who backed states' rights and racial segregation in the face of Harry S. Truman's civil rights policies.

Elected to the Senate in 1954 as a write-in Democratic candidate, Thurmond left the party in 1964 to support a Republican, Barry Goldwater, for president, a move that reflected the decline of the Democratic Party's dominance in the South.

Thurmond, who served as chairman of the armed services and judiciary committees, spoke for 24 hours against a civil rights bill in 1957 - the longest filibuster in Senate history.

But like most successful politicians, Thurmond has reinvented himself at key moments. In the late 1970s, as black voters were becoming a significant force in his state, he became one of the first Southern lawmakers to hire African-American aides and to support the appointment of blacks as federal judges.

"People want to freeze him in time, they want to freeze him in the 1940s, when he was out there making firebrand speeches, but he changed with the times," said Sen.-elect Lindsey Graham, who will succeed Thurmond when Congress convenes next month.

The transformation was complete when, during his last campaign for Senate in 1996, Thurmond was re-elected with the widest margin of African-American support of any Republican in the history of South Carolina.

"I've always had pause with it," Rep. James E. Clyburn, a black South Carolina Democrat, said of Thurmond's years as a segregationist leader. "I don't square that at all. It's part of what I work every day to overcome."

But Clyburn said he and Thurmond have become "political friends."

Armstrong Williams, a conservative black commentator who is close to the senator - for whom he interned - said he viewed Thurmond's past as a staunch foe of racial integration as merely a sign of the times.

"It was political; it was nothing that was ever in his heart," Williams said. "In his heart, he treats everyone the same."

Thurmond's popularity in his state has remained immense.

"Strom will be treated like a prophet, like a second coming" when he returns next month to his birthplace in Edgefield, Williams said.

Thurmond's electoral success in South Carolina stemmed not only from deft political maneuvering over the decades, but also from his meticulous attention to constituent service.

"If we had a problem in the family, no hesitation who to call: Strom Thurmond," Graham said. "That is his legacy back home. They say the longer you're in politics, the more people you alienate; it's been the opposite for him."

Thurmond's three children - Strom Thurmond Jr., Paul Thurmond and Julie Thurmond Whitmer - blew out the 100 candles on the cake. The senator blew out one himself.

He had a few closing words for the hundreds of well-wishers who came to mark his milestone, many of whom said they attributed the senator's longevity to his zest for life.

"May God allow you to live a long time," Thurmond said in his quiet, raspy voice, "and enjoy the time."