But West Virginia Republicans uncovered a letter Byrd had written to the imperial wizard of the KKK three years after he said he abandoned the group. In the letter, he wrote: "The Klan is needed today as never before and I am anxious to see its rebirth here in West Virginia" and "in every state in the Union."
In a 1947 letter, Byrd vowed never to fight "with a Negro by my side. Rather I should die a thousand times, and see Old Glory trampled in the dirt never to rise again, than to see this beloved land of ours become degraded by race mongrels, a throwback to the blackest specimen from the wilds."
In 1964, Byrd filibustered the landmark civil rights legislation for more than 14 hours but later said it was his biggest regret in the Senate. Decades later, he opposed the nominations of the Supreme Court's two black justices -- liberal Thurgood Marshall and conservative Clarence Thomas.
In March 2001, Byrd made headlines again after he stunned a national television audience by using the term "white niggers" when asked about the state of race relations.
"They are much, much better than they've ever been in my lifetime," Byrd said on the cable talk show. "I think we talk about race too much. There are white niggers. I've seen a lot of white niggers in my time -- I'm going to use that word. We just need to work together to make our country a better country, and I'd just as soon quit talking about it so much."
Byrd later apologized.
"The phrase dates back to my boyhood and has no place in today's society," he said.
By 2005, facing a potentially tough re-election campaign, Byrd received support from an unlikely source -- freshman Sen. Barack Obama, the only black member of the Senate, who sent out a fundraising letter on Byrd's behalf that raised nearly $825,000 in a few days.
Byrd endorsed Obama after the West Virginia primary, despite Obama's loss to Sen. Hillary Clinton, D-N.Y., in that contest.
Asked about the endorsement given his personal history on the issue of race, Byrd replied, "Those days are gone. Gone."
Despite his controversial involvement in the KKK, Byrd repeatedly apologized for it in the latter part of his career, calling it the biggest mistake of his life.
"He was in politics a very long time, and the country changed enormously between 1940 when he first ran for the state legislature and 2010," Roberts said. "He was able to adapt to the changes even as he insisted on staying firm with the role of the Senate."
Byrd served longer in the U.S. Senate than anyone else in American history and was undefeated in every election for the Senate seat he occupied.
"I served with him for 36 years. We sat in the same row," Leahy said on "GMA." "He was a senator's senator. He was a keeper of our traditions, a keeper of the rules and the kind of senator who always kept his word."
Byrd joined the Senate leadership in 1967, when his colleagues selected him to serve as secretary of the Democratic Conference. In 1971, he was chosen Senate Democratic whip.
Six years later, he was elected Democratic leader, a position he held for six consecutive terms. He served two six-year terms as both Senate majority and minority leader.