James M. Jeffords, the maverick Vermont politician who in 2001 gave Democrats a short-lived majority in the U.S. Senate when he left the Republican Party and declared himself an independent, died Aug. 18 at a retirement residence in Washington. He was 80.

Diane Derby, a former aide, confirmed his death but said she did not know the immediate cause. Mr. Jeffords declined to seek reelection to the Senate in 2006, citing his wife’s and his own declining health, and was succeeded by Bernie Sanders, his state’s longtime representative-at-large, who also is an independent.

A former Vermont state senator and attorney general, Mr. Jeffords served seven terms in the U.S. House of Representatives before winning election to the Senate in 1988. He established himself as a moderate-to-liberal Republican, a reflection of his state’s political tendencies, and frequently voted with Democrats on matters such as health care, taxes, abortion, gay rights, gun control and the environment.

He had long considered a party change, he said, without making the move. But his alienation worsened in the later years of his career — a shift attributed, depending on the source, to factors such as genuine philosophical disagreements or a desire for greater influence on behalf of his state.

On May 24, 2001, Mr. Jeffords announced that he would become an independent and caucus with Democrats. Before his move, the Senate was split 50-50 between Republicans and Democrats, with Republican Vice President Dick Cheney casting the deciding vote. Suddenly, Democrats had a one-seat advantage.

“Increasingly, I find myself in disagreement with my party,” Jeffords said at the time. “I understand that many people are more conservative than I am, and they form the Republican Party. Given the changing nature of the national party, it has become a struggle for our leaders to deal with me and for me to deal with them.”

That struggle had been long. In 1981, while serving in the House, he was the only Republican to oppose President Ronald Reagan’s tax cuts. Later, as a member of the Senate, Mr. Jeffords opposed President George H.W. Bush’s nomination of Clarence Thomas to the U.S. Supreme Court and publicly agonized before supporting the president on the invasion of Iraq during the 1991 Persian Gulf War.

During the Democratic administration of President Bill Clinton, Mr. Jeffords broke with his party by backing the president’s health-care plan and voting against the articles of impeachment brought against him in connection with the Monica Lewinsky affair. Clinton called Mr. Jeffords his “favorite Republican.”

His disaffection with his party — as well as his party’s disaffection with him — intensified after President George W. Bush’s election in 2000.

In Bush’s early days in the White House, Mr. Jeffords argued for a reduction in the proposed $1.6 trillion in tax cuts, a signature White House initiative, and strenuously pushed for billions of dollars in special education funding.

The ensuing negotiations among Mr. Jeffords, White House officials and Senate leaders were rehashed and quarreled over in the news media, with the consensus that they did not go well. At one point, according to an account in the book “Days of Fire” by journalist Peter Baker, Mr. Jeffords told Bush that he would be “a one-term president if he didn’t go beyond the conservative Republican base” on matters such as education.

“You are the swing vote,” the president replied. “You can have enormous influence.”

Shortly thereafter, Mr. Jeffords announced his party change.

Critics cast him as a turncoat and a traitor to voters who had sent him to the Senate as a Republican three times. His decision snatched committee chairmanships from Republican colleagues and booted Sen. Trent Lott, the Mississippi Republican, as majority leader. Sen. Thomas A. Daschle, Democrat from South Dakota, took his place.

Lott, a friend who had performed with Mr. Jeffords in the Singing Senators musical ensemble, criticized the move as a “ coup of one that subverted the will of the American voters who elected a Republican majority.”

“I know that it has hurt some people who may never speak to me again,” Mr. Jeffords told The Washington Post shortly after his break. “But I hope they do. It will take time. . . . It makes life tougher for them.”

As a Republican, Mr. Jeffords had chaired the Health, Education, Labor and Pensions Committee, where he frequently worked with Sen. Edward M. Kennedy (D-Mass.), the ranking Democrat, but supported Republicans and opposed unions by backing worker-management consultation in labor disputes.

In the Democratic majority, Mr. Jeffords became chairman of the Committee on Environment and Public Works. He opposed the Bush administration by voting against the resolution to authorize the war in Iraq.

In the November 2002 elections, Republicans regained their control of the chamber. The GOP increased its majority in 2004 before losing it again in 2006, the year Mr. Jeffords retired.

“I had to be true to what I thought was right,” he wrote in “My Declaration of Independence ,” a book published in 2001, “and leave the consequences to sort themselves out.”

James Merrill Jeffords was born May 11, 1934, in Rutland, Vt., to a prominent Republican family. His father, Olin M. Jeffords, was chief justice of the Vermont Supreme Court.

Mr. Jeffords received a bachelor’s degree from Yale University in 1956 and a law degree from Harvard University in 1962. He was a Navy veteran and retired from the Navy Reserve as a captain.

Mr. Jeffords practiced law before winning election to the Vermont state senate in his early 30s and as the state’s attorney general shortly thereafter. He won his House seat in 1974 and succeeded Sen. Robert Stafford, a Republican, in the Senate 14 years later. Mr. Jeffords was active in agricultural legislation important to Vermont, which has a significant dairy industry.

Mr. Jeffords’s wife, the former Elizabeth Daley, whom he married in 1961 and later divorced and remarried, died in 2007. Survivors include two children, Laura Jeffords and Leonard Jeffords, both of the District; and two grandchildren.

Once, writing in The Post, Mr. Jeffords reflected on his decision to part ways with Republicans and cited the famous poem by Robert Frost about the road not taken.

“Frost,” Mr. Jeffords observed, “does not specify precisely what difference his choice made, only that he would not choose to turn back. I feel the same way.”