At first Byrd was believed to be suffering from heat exhaustion and severe dehydration, but other medical conditions developed. He had been in frail health for several years.
Byrd, a Democrat, was the longest-serving senator in history, holding his seat for more than 50 years. He was the Senate's majority leader for six of those years and was third in the line of succession to the presidency, behind House Speaker Nancy Pelosi. 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.
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.
Yet there was nothing particularly courtly about Byrd's pursuit or exercise of power.
Byrd was a master of the Senate's bewildering 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.
"Bob is a living encyclopedia, and legislative graveyards are filled with the bones of those who underestimated him," former House Speaker Jim Wright, D-Texas, once said in remarks Byrd later displayed in his office.
In 1971, Byrd ousted Sen. Edward Kennedy of Massachusetts 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.
"I have tangled with him. He usually wins," former Sen. Dennis DeConcini, D-Ariz., once recalled.
DeConcini supported Byrd's bid for majority leader. "He reciprocated by helping me get on the Appropriations Committee," DeConcini said. Years later, DeConcini said, he displeased Byrd on another issue. "I didn't get on the Intelligence Committee when I thought I was up to get on it."
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 price 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.
But Byrd also 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. He often hesitated at unscripted moments. By 2009, aides were bringing him to and from the Senate floor in a wheelchair.
Though his hands trembled in later years, Byrd only recently lost his grip on power. Last November he surrendered his chairmanship of the Appropriations Committee.
Byrd's lodestar was protecting the Constitution. He frequently pulled out a dog-eared copy of it from a pocket in one of his trademark three-piece suits. He also defended the Senate in its age-old rivalry with the executive branch, no matter which party held the White House.
Unlike other prominent Senate Democrats such as 2004 presidential nominee John Kerry of Massachusetts, who voted to authorize the war in Iraq, Byrd stood firm in opposition -- and felt gratified when public opinion swung behind him.
"The people are becoming more and more aware that we were hoodwinked, that the leaders of this country misrepresented or exaggerated the necessity for invading Iraq," Byrd said.
He cited Iraq when he endorsed then-Sen. Barack Obama for the Democratic presidential nomination in May 2008, calling Obama "a shining young statesman, who possesses the personal temperament and courage necessary to extricate our country from this costly misadventure."
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, we 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, saying intolerance has no place in America. While supporting later civil rights bills, he opposed busing to integrate schools.
Byrd briefly sought the Democratic presidential nomination in 1976 and later told associates he had once been approached by President Richard M. Nixon, a Republican, about accepting an appointment to the Supreme Court.
But he was a creature -- and defender -- of Congress across a career that began in 1952 with his election to the House. He served three terms there before winning his Senate seat in 1958, when Dwight D. Eisenhower was in the White House.
He clashed with presidents in both parties and was implacably against proposed balanced budget amendments to the Constitution. "He is a fierce defender of the Senate and its prerogatives in ways that I think the founding fathers really intended the Senate to be," said one-time rival Kennedy.
In a measure of his tenacity, Byrd took a decade of night courses to earn a law degree in 1963, and completed his long-delayed bachelor's degree at West Virginia's Marshall University in 1994 with correspondence classes.
Byrd was a near-deity in economically struggling West Virginia, to which he delivered countless federally financed projects. Entire government bureaus opened there, including the FBI's repository for computerized fingerprint records. Even the Coast Guard had a facility in the landlocked state. Critics portrayed him as the personification of Congress' thirst for wasteful "pork" spending projects.
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.
Byrd's foster father was a miner who frequently changed jobs, and Byrd recalled that the family's house was "without electricity, ... no running water, no telephone, a little wooden outhouse."
He graduated from high school but could not afford college. 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.
Returning to meat cutting in West Virginia, he became popular for his fundamentalist Bible lectures. A grand dragon of 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; he later played it on the television show "Hee Haw" and recorded an album. He abandoned it only after a grandson's traumatic death in 1982 and when his shaky hands left him unable to play.
At his 90th birthday party in 2007, however, Byrd joined bluegrass band Lonesome Highway in singing a few tunes and topped off the night with a rendition of "Old Joe Clark."
After six years in the West Virginia legislature, Byrd was elected to the U.S. House in 1952 in a race in which his brief Klan membership became an issue. He said he joined because of its anti-communism.
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. It also led him to oppose laptops on the Senate floor and to object when a blind aide tried bringing her seeing-eye dog into the chamber.
In 2004, Byrd got Congress to require schools and colleges to teach about the Constitution every Sept. 17, the day the document was adopted in 1787.