Washington - Senator Robert C. Byrd, a son of West Virginia coal country who used his mastery of Senate rules and a taste for hardball tactics to become a passionate and often feared advocate for the state and the Senate he loved, died Monday at age 92.

The Democrat's 51 years in the Senate made him the longest serving senator in history, while his white mane, stentorian voice and flamboyant speeches citing Roman emperors gave him the presence of a man from a grander, distant time.

In many ways, Byrd embodied the changes the nation has undergone in the past half century. A one-time segregationist and opponent of civil rights legislation, he evolved into a liberal hero as one of the earliest, unrepentant and most vocal foes of the Iraq war and a supporter of the rights of gays to serve in the military. He was the acknowledged Senate Renaissance man, who could recite poetry by memory for hours and yet be ruthless in advancing his legislative agenda - which often involved corralling federal dollars for his perpetually struggling state.

As the Senate opened Monday, Byrd's desk was draped in black cloth with a bowl of white roses. Flags outside the White House and the Capitol flew at half-staff.

Senators who came to the floor to pay tribute recognized both his longevity - he served longer and cast more votes than any senator in history - and the tenacity in which he defended the traditions and prerogatives of the Senate.

"Senator Byrd's ambition was legendary," said Senate Majority Leader Harry Reid, D-Nev., recalling how shortly after he first took his oath of office on January 3, 1959, the same day Alaska became a state, he told a local newspaper that he wanted to someday chair the Senate Appropriations Committee. "Thirty years later, he was - and then lived and served for 21 more."

In many ways a throwback to an era of powerful orators such as Henry Clay or John C. Calhoun, the stiff and formal Byrd could speak at great length with fire and passion, mixing references to the Roman Empire with emotional memories of his almost seven decades with his late wife, Erma.

Brandishing his copy of the U.S. Constitution that he always carried with him, he resisted any attempt to diminish the role of the Senate, as in the days leading up to the 2003 invasion of Iraq when he was one of the few to stand up against ceding warmaking powers to President George W. Bush.

Byrd was equally tireless in steering federal dollars to his state, one of the nation's poorest, and his efforts will live on in the many highways and buildings in West Virginia that carry his name.

President Barack Obama said the Senate "has lost a venerable institution, and America has lost a voice of principle and reason."

"He held the deepest respect of members of both parties, and he was generous with his time and advice, something I appreciated greatly as a young senator," Obama said in a statement.

A spokesman for the family, Jesse Jacobs, said that Byrd died at about 3 a.m. at Inova Hospital in Fairfax, Va., where he had been since late last week. Byrd had been in frail health for several years.

Byrd was the Senate's majority leader for six of the 51 years he served there and he was third in the line of succession to the presidency, behind Vice President Joe Biden and House Speaker Nancy Pelosi. In its first order of business Monday, 85-year-old Daniel Inouye, D-Hawaii, now in his eighth term, was sworn in to replace Byrd in the mostly honorary position of president pro tempore of the Senate.

Tributes to the Senate's dean lent a somber tone to the first day Monday of Senate Judiciary Committee confirmation hearings for Supreme Court nominee Elena Kagan.

"No senator came to care more about the Constitution and be a more effective defender of our constitutional government than the senior senator from West Virginia," Committee Chairman Patrick Leahy, D-Vt., said in his opening remarks. "In many ways, he was the keeper of the Senate flame, the fiercest defender of the Senate's constitutional role and prerogatives."

Separately Sen. Jay Rockefeller, a fellow West Virginian in the Senate, said it was his "greatest privilege" to serve with Byrd.

"I looked up to him, I fought next to him, and I am deeply saddened that he is gone," Rockefeller said.

The Senate Republican leader, Mitch McConnell of Kentucky, said Byrd "combined a devotion to the U.S. Constitution with a deep learning of history to defend the interests of his state and the traditions of the Senate."

"We will remember him for his fighter's spirit, his abiding faith, and for the many times he recalled the Senate to its purposes," McConnell said.

Former President Jimmy Carter said Byrd "was my closest and most valuable adviser" during his presidency, when Byrd served as Senate majority leader. Byrd was instrumental in getting the votes to pass the Panama Canal treaty Carter wanted, overcoming strong Republican opposition.

Byrd was skilled "in using arcane Senate rules to achieve his goals, and was proud of his ability to count votes and forge prevailing coalitions," Carter said in a statement.

West Virginia Gov. Joe Manchin, a Democrat, will appoint Byrd's replacement. Democratic sources, who spoke on condition of anonymity because they were not authorized to disclose confidential conversations, said Manchin has told associates in the past he was interested in the seat.

The governor issued a statement Monday saying Byrd "was a fearless fighter for the Constitution, his beloved state and its great people." He told The Associated Press that he will not appoint himself to fill the seat, and had no timetable for naming a replacement.

In comportment and style, Byrd often seemed a Senate throwback to a courtlier 19th century. He could recite poetry, quote the Bible, discuss the Constitutional Convention and detail the Peloponnesian Wars - and frequently did in Senate debates.

In 1971, Byrd ousted Sen. Edward Kennedy, the Massachusetts senator, as the Democrats' second in command. He was elected majority leader in 1976 and held the post until Democrats lost control of the Senate four years later. He remained his party's leader through six years in the minority, then spent another two years as majority leader.

Byrd stepped aside as majority leader in 1989 when Democrats sought a more contemporary television spokesman. "I ran the Senate like a stern parent," Byrd wrote in his memoir, "Child of the Appalachian Coalfields." His consolation prize was the chairmanship of the Appropriations Committee, with control over almost limitless federal spending.

Within two years, he surpassed his announced five-year goal of making sure more than $1 billion in federal funds was sent back to West Virginia, money used to build highways, bridges, buildings and other facilities, some named after him.

In 2006 and with 64 percent of the vote, Byrd won an unprecedented ninth term in the Senate just months after surpassing South Carolinian Strom Thurmond's record as its longest-serving member. His more than 18,500 roll call votes were another record.