(CNN) -- Jim Jeffords, who shifted the balance of power in the Senate when he bolted from the Republican Party in 2001, died on Monday.
The former Vermont lawmaker had been in declining health and died at a military retirement center in Washington, according to a one-time aide, Diane Derby.
He was 80.
Jeffords was a veteran member of the House before moving to the Senate in 1989, where he was one of the most liberal Republicans. Jeffords embraced issues like education and the environment.
Years later, many in the party believed he single-handedly sank a sweeping tax cut package proposed by then-President George W. Bush, forcing the White House to negotiate a smaller deal with Democrats.
Political tensions boiled over and Jeffords became an independent in 2001 and caucused with Democrats. The drastic step shifted Senate control to Democrats. He left the Senate in 2007.
President Barack Obama said in a statement that Jeffords never lost the "fiercely independent spirit that made Vermonters, and people across America, trust and respect him" and that he "voted his principles, even if it sometimes meant taking a lonely or unpopular stance."