WASHINGTON - Sen. Robert C. Byrd, 92, 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.

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, Sen. Byrd embodied the changes the nation has undergone in the past half century. A onetime 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.

In many ways a throwback to an era of powerful orators like Henry Clay or John C. Calhoun, the stiff and formal Sen. 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. Sen. Byrd was equally tireless in steering federal dollars to his state, one of the nation's poorest.

Sen. 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 Biden and House Speaker Nancy Pelosi. Sen. Daniel Inouye, 85, of Hawaii, now takes the role of Senate president pro tempore as the person with the most seniority.

A spokesman for the family said that Sen. Byrd died about 3 a.m. at Inova Hospital in Fairfax, Va. He had been in frail health for several years.

President Obama said Monday that 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.

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," said Committee Chairman Patrick Leahy (D., Vt.). "In many ways, he was the keeper of the Senate flame, the fiercest defender of the Senate's constitutional role and prerogatives."

Senate Republican leader Mitch McConnell of Kentucky, said Sen. 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."

West Virginia Gov. Joe Manchin, a Democrat, will appoint Sen. Byrd's replacement.

In comportment and style, Sen. 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.

Yet there was nothing particularly courtly about Sen. Byrd's pursuit or exercise of power. He was a master of Senate rules and longtime chairman of the Senate Appropriations Committee, which controls a third of the $3 trillion federal budget. He was willing to use both to reward friends and punish those he viewed as having slighted him.

In 1971, Sen. Byrd ousted Sen. Edward M. Kennedy (D., Mass.) 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. Sen. Byrd stepped aside as majority leader in 1989 when Democrats sought a more contemporary television spokesman.

"I ran the Senate like a stern parent," Sen. Byrd wrote in his memoir, Child of the Appalachian Coalfields.

In 2006 and with 64 percent of the vote, Sen. 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.

Sen. Byrd seemed to slow after the death of Erma, his wife of almost 69 years, in 2006. Frail and at times wistful, he used two canes to walk haltingly and needed help from aides to make his way about the Senate. In late 2008 he surrendered his chairmanship of the Appropriations Committee.

Sen. Byrd's accomplishments followed a childhood of poverty in West Virginia, and his success on the national stage came despite a complicated history on racial matters. As a young man, he was a member of the Ku Klux Klan for a brief period, and he joined Southern Democrats in an unsuccessful filibuster against the landmark 1964 Civil Rights Act. He later apologized for both actions.

Robert Carlyle Byrd was born Nov. 20, 1917, in North Wilkesboro, N.C., as Cornelius Calvin Sale Jr., the youngest of five children.

Before he was 1, his mother died and his father sent him to live with an aunt and uncle, Vlurma and Titus Byrd, who renamed him and moved to the coal-mining town of Stotesbury, W.Va. He didn't learn his original name until he was 16 and his real birthday until he was 54.

He graduated from high school but could not afford college. In a measure of his tenacity later in life, he took a decade of night courses to earn a law degree in 1963, and completed his degree in 1994 with correspondence courses.

Married in 1936 to high school sweetheart Erma Ora James, with whom he had two daughters, he pumped gas, cut meat and during World War II was a shipyard welder.

He became popular for his fundamentalist Bible lectures and the Ku Klux Klan suggested he run for office. He won his first race for the state's House of Delegates in 1946, distinguishing himself from 12 rivals by singing and fiddling mountain tunes. His fiddle became a fixture.

Sen. Byrd entered Congress as one of its most conservative Democrats. He was an early supporter of the Vietnam War, and his 14-hour, 13-minute filibuster against the 1964 civil rights bill remains one of the longest ever. His views gradually moderated, particularly on economic issues, but he always sided with his state's coal interests in confrontations with environmentalists. His love of Senate traditions inspired him to write a four-volume history of the chamber.

Sen. Byrd's lodestar was protecting the Constitution. In 2004, he persuaded Congress to require schools and colleges to teach about the Constitution every Sept. 17, the day the document was adopted in 1787.