A few months later, Byrd acknowledged that the Senate -- in which he said he was "deeply disappointed" -- would give Bush the authority for war with Iraq.
In that speech, he repeatedly referred to values in the Constitution: "Those values do not include striking first at other countries, at other nations. Those values do not include using our position as the most formidable nation in the world to bully and intimidate other nations."
Byrd warned that after the invasion, "a second war, a war to win the peace in Iraq," could cost hundreds of billions of dollars -- a view not taken seriously by many in the buildup to the war. He also railed against what he viewed as the United States' loss of the moral high ground as a result of the Iraq invasion, and kept up the drumbeat as a majority of Americans' opinion turned against the war.
In his final years, Byrd was also more likely to challenge the coal industry in his home state. Last December, he said that the industry must change.
"Change has been a constant throughout the history of our coal industry. West Virginians can choose to anticipate change and adapt to it, or resist and be overrun by it," Byrd said. "The time has arrived for the people of the Mountain State to think long and hard about which course they want to choose."
After the explosion that killed 29 men at the Upper Big Branch Mine in Raleigh County in April, he said the mine's owner, Massey Energy, and federal mine regulators both "have much to explain." Earlier this month, he voted against a bill that would have overturned a finding by the Environmental Protection Agency that greenhouse gas emissions pose a public health threat.
As he entered this tenth decade, Byrd's hands frequently shook, he had difficulty walking and he delegated more of his responsibilities to other senators and staff members. His public appearances, less frequent, were in a wheelchair.
His wife of nearly 70 years, Erma, died in 2006. Washington rumors ran rampant shortly afterward that Senate leaders were making plans to move him out of his leadership posts.
But Byrd's response was spirited to those who said he was too old to perform his duties.
"In a culture of Botox, wrinkle cream and hair dye, we cannot imagine that becoming older is a good thing, an experience to look forward to and a state worthy of respect," he said after a national story about his advanced age in 2007.
He was hospitalized three times in 2008 and twice more in 2009, including a 47-day stay in late spring.
In November 2008, Byrd finally agreed to give up the chairman's post on the Senate Appropriations Committee. On the day he gave up the chairmanship, his office announced that a $32 million Veterans Affairs data center would be built in Martinsburg.
He said he stepped down only after he was "confident that stepping aside as chairman will not adversely impact my home state of West Virginia."
Byrd was born Cornelius Calvin Sale Jr. on Nov. 20, 1917, in North Wilkesboro, N.C., the youngest of five children. His mother, Ada Kirby Sale, died in an influenza epidemic on Nov. 10, 1918, the day before World War I ended. At her request, the boy's father, a factory worker, sent him to be raised by an aunt and her husband, Titus and Vlurma Byrd.
The couple, who had no children of their own, renamed the boy Robert and moved to Bluefield, and later to Raleigh County. Byrd's uncle worked at several jobs during the Great Depression, including coal miner, brewery worker and farmer.
Byrd graduated as valedictorian from Mark Twain High School in Stotesbury, Raleigh County, in 1934. As a teenager, he also learned to play the fiddle. (The instrument became a constant companion on Byrd's West Virginia travels, and Byrd released an album, "Mountain Fiddler," in 1978.)
He married his high school sweetheart, Erma Ora James, a coal miner's daughter, in 1937. In his autobiography, Byrd wrote that they were married by a "hard-shell Baptist preacher" with their parents in attendance. That night, he wrote, "we went to a square dance, about the only thing happening on Saturday nights. I played the fiddle and Erma danced."
Byrd's first job was at a gas station in Helen, followed by a turn as a "produce boy" at a store in Stotesbury. During World War II, he worked as a shipyard welder in Baltimore and Tampa.
He, Erma and their two daughters returned to Raleigh County after the war, and he won a race among 14 Democrats for the state House of Delegates in 1946. A couple of years later, the Byrds opened their own store in Sophia, a town where Byrd claimed residence for more than 60 years.
Byrd was elected to the state Senate in 1950, the U.S. House of Representatives in 1952 and, in 1958, to the first of a record nine terms in the U.S. Senate.
He was selected as the Sunday Gazette-Mail's West Virginian of the Year four times: in 1974, 1977, 1990 and 2002.
In addition to his autobiography, Byrd was the author of a four-volume history of the U.S. Senate and a history of the Roman Senate. He also penned the 2004 book "Losing America: Confronting a Reckless and Arrogant Presidency" and the 2008 work "Letter to a New President."
According to his biography on the Senate's website, Byrd is survived by two daughters and their husbands, Mona and Mohammad Fatemi and Marjorie and Jon Moore; five grandchildren, Erik Fatemi, Darius Fatemi, Fredrik Fatemi, Mona Pearson and Mary Anne Clarkson; five great-granddaughters and two great-grandsons. Another grandson, Michael Moore, was killed in a 1982 traffic accident.