Byrd's spokesperson Jesse Jacobs said the 92-year-old senator was initially admitted to a Washington-area hospital late last week for what doctors thought was a heat-related illness.
Democratic Senator Robert Byrd, the longest-serving member of the US Congress in history, is seriously ill and has been hospitalised, a spokesperson for the West Virginia lawmaker said on Sunday.
Byrd's spokesperson Jesse Jacobs said the 92-year-old senator was initially admitted to a Washington-area hospital late last week for what doctors thought was a heat-related illness.
"However, upon further examination by his doctors, other conditions have developed which has resulted in his condition being described as 'serious,'" Jacobs said in a statement.
Byrd was first elected to the House of Representatives in 1952 and served six years there before moving to the Senate.
Until last year, the often-ailing senator was the top Democrat on the powerful Appropriations Committee. He has also served as the Senate's Democratic leader and held other key positions.
Byrd was an early and outspoken opponent of the Iraq war that began in 2003 and also warned against a buildup of US troops in Afghanistan.
A former member of the Ku Klux Klan white supremacist group, Byrd later became a champion of civil rights. He has also been a staunch supporter of his home state''s coal industry and more recently has spoken out about environmental and safety issues.
The senator's health has been failing for years, often forcing him to use a wheelchair. He was hospitalized last September after falling.
Byrd's illness could hurt Democrats' chances of getting the needed 60 votes in the 100-member Senate to clear a Republican procedural roadblock against the new financial regulation bill crafted last week by House and Senate negotiators.
Democrats voiced confidence last week that the Senate would pass the measure, but it was not immediately clear if they could do so without Byrd.
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