Byrd helped shape much of the nation's history and served with a dozen US presidents. He died peacefully at Inova Fairfax Hospital outside of Washington, DC.
US senator Robert Byrd, who evolved from a segregationist to a civil rights advocate in becoming the longest serving member ever of the Congress, died today, a spokesperson for the West Virginia Democrat said.
First elected to Congress in 1952, Byrd was 92.
Byrd helped shape much of the nation's history and served with a dozen US presidents. He died peacefully at Inova Fairfax Hospital outside of Washington, DC, said his spokesperson, Jesse Jacobs. Byrd was hospitalized last week with what doctors believed was a heat-related illness.
"I love to serve. I love the Senate. If I could live another 100 years, I'd like to continue in the Senate," Byrd, who kept a copy of the US Constitution in his breast pocket, said in a 2006 interview.
Byrd's death is not expected to have any immediate impact on the Democrats' 59-41 control of the Senate.
West Virginia governor Joe Manchin is virtually certain to appoint a fellow Democrat to succeed Byrd, whose current term expires in 2012.
Senator Jay Rockefeller, also of West Virginia, said: "senator Byrd came from humble beginnings in the southern coalfields ... and triumphantly rose to the heights of power in America. But he never forgot where he came from nor who he represented, and he never abused that power for his own gain."
Senate Republican leader Mitch McConnell said Byrd will be remembered "for his fighter's spirit, his abiding faith and for the many times he recalled the Senate to its purposes."
Byrd was first elected to the House of Representatives in 1952, and served six years in that chamber before moving to the Senate. His early campaigns were punctuated by his skills as a bluegrass fiddler that helped draw big and enthusiastic crowds for the self-described West Virginia "hillbilly."
With his old-fashioned courtliness, Byrd was a defender of the Senate's traditions and over the years held most of its key positions, including Democratic leader from 1977 to 1988 and later as the top Democrat on the powerful Appropriations Committee.
Byrd was an early and eloquent opponent of the Iraq war, which began in 2003 with popular support but within a few years was widely condemned. He also warned against a buildup of US troops in Afghanistan.
He worked with and challenged presidents, Democrats and Republicans alike, and reminded all of them of Congress' responsibility to check their power.
"I'm not any president's man. I'm a Senate's man," Byrd said in the 2006 interview.
During his more than half century in Congress, America changed dramatically and so did Byrd.
"When I got here, I was to the right of Barry Goldwater," Byrd said, referring to a conservative Republican senator and failed 1964 presidential candidate. "I moved more to the center."
In the early 1940s, before being elected to Congress, Byrd belonged to the Ku Klux Klan, a membership that he attributed to youthful indiscretion.
"It has emerged throughout my life to haunt and embarrass me and has taught me in a very graphic way what one major mistake can do to one's life, career and reputation," Byrd wrote in a 1987 memoir, "Robert C. Byrd: Child of the Appalachian Coalfields."
In Congress, Byrd, who denounced civil rights leader Martin Luther King as a "self-seeking rabble rouser," eventually became a leading backer of civil rights.
Of the record-setting 18,500-plus Senate votes Byrd cast, he said his biggest regret was opposing the 1964 Civil Rights Act, a landmark law that brought down barriers for black Americans.
He said his views changed most dramatically after his teenage grandson was killed in a 1982 traffic accident that Byrd said put him in a deep emotional valley.
"The death of my grandson caused me to stop and think," Byrd said. "I came to realize that black people love their children as much as I do mine."
In West Virginia, Byrd was revered for his ability to deliver federal dollars to his poor state to build roads, schools and hospitals.
Critics called him the "Prince of Pork," but constituents crowned him as "West Virginian of the 20th Century."
"I want to be West Virginia's billion dollar industry," Byrd declared in 1990. He succeeded.
In 2000 he won passage of a bill that took import duties paid by foreign firms and transferred them to US corporations. The Byrd Amendment, reviled abroad, was aimed at helping ailing steel companies in West Virginia and other states.
He also was a long-time champion of his state's coal industry, drawing the frequent ire of environmentalists, but later became more conscious of environmental damage and shortcomings in worker safety.
Byrd was born Cornelius Calvin Sale Jr. on Nov. 20, 1917, in North Carolina and was sent to live with relatives in West Virginia after his mother died in the 1918 flu pandemic.
His new family renamed him and Byrd grew up desperately poor in the West Virginia coal fields. Unable to afford college, he worked as a meat cutter during the Great Depression and later as a welder building ships during World War Two.
Byrd married his high school sweetheart, Erma Ora James, in 1936. They had two daughters and six grandchildren.
"For two hillbillies -- that is what we are, two hillbillies -- from West Virginia, it has been an exciting and wild ride," Byrd said in a Senate speech marking their 65th anniversary. She died in March 2006.
Byrd set the record for congressional longevity on Nov. 17, 2009, eclipsing the mark previously held by the late Carl Hayden.
On that day, Byrd said, "My only regret is that my beloved wife, companion and confidant, my dear Erma, is not here with me. I know that she is looking down from the heavens smiling at me and saying, ''Congratulations, my dear Robert -- but don't let it go to your head.''