Sen. Jim Jeffords (search), an independent who triggered one of the most dramatic upheavals in U.S. Senate history when he quit the GOP (search) four years ago, announced Wednesday he would retire at the end of his term next year, citing his and his wife's health.

Jeffords, 70, had been adamant in saying he would seek re-election, but he told reporters he would not seek a fourth term.

"After much thought and consultation with my family and staff, I have decided to close this chapter of my service to Vermont and not seek re-election in 2006," Jeffords said at a news conference at a suburban Burlington hotel.

Jeffords has suffered from a bad back and neck for years and has seemed confused by some of the questions in several recent news interviews.

But it was the health of his wife, Liz, he highlighted in his remarks.

"Liz, as you know, is battling cancer and will soon have to undergo another round of chemotherapy," he said.

As for his own situation, he said, "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. My memory fails me on occasion, but Liz would probably argue this has been going on for the last 50 years."

His spokesman, Erik Smulson, said just over a week ago that Jeffords was in excellent health and looking forward to "a spirited campaign."

Jeffords' surprise decision was likely to unleash a host of candidates to replace him in the predominantly Democratic state. U.S. Rep. Bernie Sanders (search), the state's only congressman and the only independent in the House, had said he would run if Jeffords did not. Like Jeffords, Sanders votes with the Democrats. Vermont's other member of the Senate is Democrat Patrick Leahy (search).

Republicans Richard Tarrant, a business executive, and Gov. James Douglas also are possible contenders.

Former Gov. Howard Dean, now the chair of the Democratic National Committee, had long been rumored as a possible candidate. But Karen Finney, a top aide to Dean, said the former Vermont governor and 2004 presidential hopeful will not run for the Senate because he's committed to his new duties at the DNC.

Jeffords was elected as a Republican to the U.S. House in 1974 and to the U.S. Senate in 1988, but he abandoned the party in 2001 because of disagreements with the Bush administration.

"In order to best represent my state of Vermont, my own conscience and principles that I have stood for my whole life, I will leave the Republican Party and become an independent," said Jeffords on May 24, 2001.

At the time the Senate was evenly divided, and Jeffords' decision to become an independent and caucus with the Democrats put the Democrats in charge of the Senate. They retained power until after the 2002 elections.

Jeffords continues to vote with the Democrats and was a major fundraiser for them last fall.

The GOP now holds 55 Senate seats to 44 Democrats, with Jeffords as the lone independent.

He has hired campaign staff and has more than $2 million in his campaign account.