Updated: 6:02 a.m.

Sen. Robert Byrd (D-W.Va.), the longest-serving Member in history, died at 3 a.m. Monday after a brief hospital stay. He was 92.

“I am saddened that the family of U.S. Senator Robert C. Byrd, D-W.Va., tearfully announces the passing of the longest serving member of Congress in U.S. history,” Byrd’s office said in a statement.

In a separate statement Sunday afternoon, his office said Byrd had been hospitalized after suffering from heat exhaustion and dehydration. He was described as being “seriously ill.”

Byrd, who was first elected to the Senate in 1958, was President Pro Tem, a largely ceremonial post but one that put him third in succession to the presidency. Daniel Inouye (D-Hawaii) is next in line for the position.

On Nov. 18, his 20,774th day of service in Congress, Byrd reached a milestone as the longest-serving lawmaker in Congressional history. He had previously held the title of the longest-serving Senator.

He also served in every Senate leadership role, including as Majority Leader from 1977 to 1981 and from 1987 to 1989.

Byrd’s health had been deteriorating: He had appeared increasingly frail and was forced to use a wheelchair for the past several years. Byrd spent several weeks in the hospital in May and June of last year, and he was rushed by ambulance to a hospital Sept. 22 after falling at his Northern Virginia home. On Sunday, his office announced he was in the care of a Washington-area hospital, where he was sent last week after suffering from heat exhaustion and dehydration.

Byrd’s appearances in the Senate were spotty over the past year. He delivered an impassioned tribute on Sept. 10 to his late colleague Sen. Edward Kennedy (D-Mass.), who died of brain cancer on Aug. 25, and he took to the well of the chamber Oct. 21 to warn against heightened U.S. involvement in Afghanistan.

He also was present in December for a dramatic vote — held just past 7 a.m. on Christmas Eve — to support President Barack Obama’s signature health care overhaul, as well as for pivotal procedural votes leading up to its passage.

Byrd’s profile rose following a deadly mining explosion in West Virginia in April, and he offered his influential opinion in the debate over the filibuster at a Rules Committee hearing in May. His most recent vote was June 10.

Byrd’s age and health started to become problematic during the 110th Congress, when Senate Democrats feared that he no longer had the vigor to lead the powerful Appropriations Committee. After subtle nudging from leadership, Byrd voluntarily ceded the gavel in the 111th Congress to Inouye, his longtime friend and colleague.

Still, over the span of his nine Senate terms, Byrd never lost the respect of his colleagues, no matter their political bent. He was a prolific legislator, an institutionalist and a passionate orator who never went anywhere without a pocket-size version of the Constitution, which he quoted regularly.

Byrd came to the Senate after serving three terms in the House. Unlike many of his Congressional colleagues, he grew up poor and had no formal education. His appetite for learning was insatiable, however, and he developed a keen knowledge of Senate rules and procedures that was virtually unmatched by any other Senator.

“Here’s someone who’s in a chamber with people who went to Harvard and Yale, and you’d think that would make him feel inferior,” Senate Historian Don Ritchie said. “But he read all the books and applied all the rules in a way the others forgot after their college years.”

Byrd delivered a series of speeches about Senate history that began in 1980 when he served as Majority Leader. Relegated to minority status in 1981, Byrd continued the speeches, which grew into a four-volume series, with the first published in 1988.

“I love this Senate. I love the Senate for its rules. I love the Senate for its precedents. I love the Senate for the difference it can make in people’s lives,” Byrd said June 21, 2007, the day he cast his 18,000th vote.

Byrd was also notorious for his tenure on the Appropriations Committee, where he steered thousands of earmarks back to his home state. Over his career, he sent more than $1 billion back to West Virginia, where he had 33 projects named after him, according to Citizens Against Government Waste. In his rural state, Byrd’s name will live on thanks to the Robert C. Byrd Federal Courthouse, the Robert C. Byrd Locks and Dam, and the Robert C. Byrd Center for Hospitality and Tourism.

“I want to be West Virginia’s billion-dollar industry,” Byrd once said.

Born Cornelius Calvin Sale Jr. in North Wilkesboro, N.C., in 1917, Byrd was renamed by his aunt and uncle, who took custody of him after his mother died in 1918. He was raised in a small coal-mining town in West Virginia and graduated at the top of his high school class. He married his high school sweetheart, Erma James Byrd, in 1937. She died in 2006.

Byrd earned high marks as a student at Marshall University, but he dropped out after one semester because he couldn’t afford the tuition. The university eventually awarded him a bachelor’s degree in 1994, when he was 77. He earned a law degree from American University in 1963, after taking classes for 10 years while serving as a Member of Congress.

Byrd was elected to the state Legislature at age 30, serving three terms, then was elected to the House in 1952 and to the Senate in 1958. Voters chose him as their Senator nine consecutive times, most recently in 2006. Byrd never lost an election, and he made West Virginia history in 1970 when he won each of the state’s 55 counties.

But Byrd’s record was not perfect. He was always haunted by his stint as a member of the Ku Klux Klan in the 1940s. Though he repeatedly expressed regret about joining the white supremacist group, his affiliation was a noted part of his biography.

“It has emerged throughout my life to haunt and embarrass me,” Byrd wrote in his 2005 memoir, “Robert C. Byrd: Child of the Appalachian Coalfields.”

Byrd supported segregation in the 1950s, opposed the Civil Rights Act of 1964 and voted against the nomination of Thurgood Marshall — the first African-American member of the Supreme Court — in 1967.

Byrd later went on to endorse then-Sen. Obama for president in 2008, calling his one-time colleague “a noble-hearted patriot and humble Christian.”

On the day that Byrd cast his 18,000th vote, he acknowledged the changing times that coincided with his Senate career. Yet he also noted that the mission of the chamber would remain the same long after he was gone.

“The Senate was viewed by the framers as a place where mature wisdom would reside,” Byrd told his Senate colleagues. “The world has changed. But our responsibilities, our duties as Senators have not changed. We have a responsibility, a duty to the people to make our country a better place.”

In more recent years, Byrd became a leading Democratic voice against the Iraq War. He took to the floor in March 2003 to speak out against the use of military force in the country, saying: “Today I weep for my country. I have watched the events of recent months with a heavy, heavy heart. No more is the image of America one of strong, yet benevolent peacekeeper. The image of America has changed. Around the globe, our friends mistrust us, our word is disputed, our intentions are questioned.”

Byrd mentored dozens of freshman Senators from both parties, including Majority Leader Harry Reid (D-Nev.), Obama and former Sen. Hillary Rodham Clinton, who is now secretary of State.

With Byrd’s death comes a loss of institutional knowledge that will undoubtedly be missed. Sen. Jay Rockefeller (D-W.Va.), elected to a fifth term in 2008, now becomes the senior Senator from the Mountain State.

Byrd is survived by daughters Mona Carole Byrd Fatemi and Marjorie Ellen Byrd Moore, five grandchildren and two great grandchildren.