SOUTH BURLINGTON, Vt. – Senator Jeffords, who single-handedly upset the balance of power on Capitol Hill four years ago when he quit the Republican Party to become an independent, announced yesterday he will retire at the end of his term next year, citing his own health problems and those of his wife.

The surprise announcement immediately triggered a scramble for Mr. Jeffords’s seat, one of several that will be up for grabs in next year’s midterm elections.

In recent months, Mr. Jeffords’s family and his staff questioned whether the 70-year-old senator was physically and mentally up to a statewide campaign for a fourth term. He stumbled in a recent radio interview, and was confused about some of his votes. His wife, Liz, is battling cancer.

“It is time to begin a new chapter, both for me personally and for the people of Vermont,” said Mr. Jeffords, who also has suffered from a bad back and neck. “There have been questions about my health, and that is a factor as well. I am feeling the aches and pains when you reach 70.”

Mr. Jeffords’s retirement will bring an end to a three-decade career in Washington. He won election to the House in 1974 as a Republican and moved to the Senate in 1988.

In 2001, he abandoned the GOP and aligned himself with the Democrats, putting them in control of the evenly divided Senate. The switch made him a hero among Democrats and a traitor among Republicans.

The Senate Republican leader at the time, Mississippi’s Trent Lott, dubbed Mr. Jeffords’s action a “coup of one,” and described it as “the impetuous decision of one man to undermine our democracy.”

At the heart of Mr. Jeffords’s decision was a belief that the GOP in general and President Bush in particular had become too conservative and that he could not remain in a party that favored tax breaks for the wealthy over full funding of education programs for the disabled.

He complained at the time that the Republicans in control of both the White House and Congress “were set out on an agenda that did not fit into what the average American wanted to see.”

The Democrats’ control of the Senate was brief. Republicans took it back 18 months later and added to their gains in last fall’s election. They now hold 55 seats out of 100.

Still, Mr. Jeffords has become a hero to Democrats in the four years since. He has been one of the party’s biggest fund-raisers, attracting huge crowds as he traveled the country helping to bring in millions in 2002 and 2004 for Democratic candidates.

His 2006 re-election seemed all but assured in predominantly Democratic Vermont: He had $2 million in the bank, had hired a campaign staff, and had won the endorsements of state Democratic leaders, including Senator Leahy and Howard Dean, the former governor, who is now chairman of the Democratic National Committee. Mr. Jeffords was said to be eager to run for re-election to show Republicans that Vermont would elect him as an independent.

The chairman of the state GOP recently sent out a national fund-raising letter that branded Mr. Jeffords a “turncoat” and called his 2001 decision “despicable.” “He betrayed President Bush and ALL Republicans,” wrote Jim Barnett. “Now it is time for payback.”

Mr. Jeffords’s spokesman Erik Smulson said the senator made the decision not to run over the weekend.

“I have had an enormously satisfying career, one that I would not have traded for any other,” Mr. Jeffords said, his wife and two adult children at his side. “In no other job do you have both the freedom and obligation to solve problems and help people on a daily basis.”

Within an hour of the announcement, Vermont’s sole congressman, independent Bernie Sanders, all but declared himself a candidate for Jeffords’ seat. Governor Douglas, the state’s top Republican, is another possible candidate.