Jim Jeffords, the former senator from Vermont who single-handedly redrew the national political map in 2001 when, after a quarter-century as a moderate Republican lawmaker, he declared himself an independent, shifting control of the Senate to the Democrats, died on Monday in Washington. He was 80.

The cause was complications of pneumonia, his son, Leonard, said.

Vermont’s lone congressman from 1975 until his election to the Senate in 1988, Mr. Jeffords was a solid Republican on military issues. But as early as 1981, when he voted against President Ronald Reagan’s package of tax and budget cuts — the only House Republican to do so — he showed a disinclination to be bound by his party’s conservative orthodoxy.

A supporter of abortion rights, gay rights and the National Endowment for the Arts — left-leaning stances perhaps befitting an elected representative of a state that had become one of the nation’s bluest — he was in favor of the health care plan proposed by President Bill Clinton and opposed Mr. Clinton’s impeachment. He backed legislation promoting environmental protection, funding for education and aid for the disabled. He voted against President George Bush’s nomination of Clarence Thomas to the Supreme Court.

But many of his Republican colleagues were shocked when, after the election of George W. Bush in 2000, his displeasure with the further rightward shift of the party caused him to abandon it and to caucus with the Democrats as an independent.