Born November 20, 1917, Byrd had been known as the longest serving member of Congress in the history of the United States with more than 50 years in his seat representing his home state.

He spent much of that time ensuring that regulations for coal mines across the nation were properly enforced.

Born Cornelius Calvin Sale Jr, he was raised in the coalfields of southern West Virginia after his mother died in the 1918 flu pandemic and he was adopted by his aunt and uncle.

Byrd, who attended Beckley College, Concord College, Morris Harvey College, and Marshall College, was initially elected to the United States House of Representatives in 1952, where he served for six years. He was elected to the Senate in 1958.

He went on to be Senate majority leader from 1977 to 1981, and 1987 to 1989, and minority leader from 1981 to 1987.

While president pro tempore, he was third in the presidential succession line behind the US vice president and speaker of the house.

US House majority Whip Mike Caputo said Monday following the announcement of Byrd’s passing that he held great admiration for his fight in keeping miners safe.

“Senator Byrd’s service to West Virginia and the country was invaluable in countless respects, but the coal miners of West Virginia in particular owe him a huge debt of gratitude,” he said.

“As UMWA District 31 international vice president and a former coal miner, I deeply appreciate his concern for and dedication to miners both in West Virginia and throughout the country. Senator Byrd was the best friend the coal miner has ever had.”

State Governor Joe Manchin also expressed sadness on behalf of the state after news of the passing, saying the news “broke our hearts”

“Senator Byrd was a fearless fighter for the constitution, his beloved state and its great people. He made a significant mark as a member of Congress in both our state's and nation's history [and] his accomplishments and contributions will define history for eternity.”

Reaction came pouring in from across the coal industry he had fought so hard to support, including Appalachian producers such as Massey Energy and West Virginia-based pro-coal group FACES of Coal, who called Byrd a “tireless worker” for the best interests of the state and entire US.

"He was a man who was raised in the coal fields and was a tireless worker in trying to make miners safer, often asking the country to appreciate West Virginia's greatest natural resources, coal and the coal miner,” FACES coordinator Bryan Brown said.

“The news of Senator Byrd's passing brings sadness to all West Virginians. The Senator's service to this great state and our nation will always be appreciated and remembered."

Department of Labor secretary Hilda Solis, who worked with Bryd often on issues relating to mining regulations and miner safety, also released a statement Monday afternoon.

"Few people ever really earn the right to be called a legend; however, Senator Robert Byrd earned it every day,” she said.

"I had the pleasure of working with [him] on issues that impacted our nation's coal miners and their families. As the son of a coal miner, Senator Byrd understood that we have a moral obligation to protect the men and women who go underground every day to dig the coal that fuels our economy, and to ensure that those men and women return safely to their loved ones after every shift.”

The nation’s largest mining union also said it was mourning the loss of a man who was a driving force in the passage of the landmark Coal Miner Health and Safety Act legislation of 1969.

“The United Mine Workers and all coal-mining families and communities have lost their best friend,” United Mine Workers of American international president Cecil Roberts said.

“He dedicated his entire life to making things better for coal miners everywhere.

He noted that in the 40 years prior to the passage of the 1969 Mine Act, 32,000 miners died in mining incidents, but in the 40 years since that number stands at just over 3200.

“One could argue that thanks to Senator Byrd’s efforts, 29,000 lives were saved.”

Roberts noted that Byrd was also a constant fighter for more miner protection, including a large role in securing revisions to the 1969 Act in 1977.

He rallied for black lung benefits for a long time and continued healthcare benefits for mining families in 1992 and many times since.

He also pushed for answers to the Aracoma and Sago disasters of 2006, but his work did not stop there.

“Despite failing health, Senator Byrd fought to the very end to improve miners’ lives,” Roberts said.

“Last spring, his amendments to the health care bill restored the process of miners getting – and their widows keeping – black lung benefits for those miners who have been totally disabled by this terrible disease. And just last month, he grilled Massey CEO Don Blankenship during a Senate hearing he led into the Upper Big Branch mine disaster.”

American Federation of Labor and Congress of Industrial Organizations president Richard Trumka said its industry membership would mourn Byrd’s death with sadness but remember his life, work and legacy with joy.

"Senator Byrd's record in the Senate was unabashedly and unapologetically on the side of working families – as majority leader he helped lead the fight against the Republican filibuster of labor law reform in the 1970s,” he said.

“He tirelessly fought for health and safety laws that protected workers, opposed job-killing trade deals and, when it came to standing up to the coal companies, a miner never had a stronger ally.”