BURLINGTON, Vt. - It was an improbable spectacle: a man so unassuming he is known as "Geesum Jim," from a state of no consequence in American politics, changes the course of the most powerful government on earth by simply stating he no longer believes in the party he grew up in.

That is precisely what happened yesterday morning on the shores of Lake Champlain as U.S. Sen. Jim Jeffords stood in a hotel ballroom and explained why he was leaving the Republican Party.

"I was not elected to this office to be something I am not," Jeffords said.

It was not, Jeffords said, that he was leaving the Republican Party. It is that the Republican Party had careened so far to the right that it had abandoned him, his beloved Vermont and, by extension, most of the rest of New England.

The scene was out of an election night victory party — sans the champagne — with Jeffords treated as a conquering hero by several hundred supporters who crowded the Radisson Hotel, waving signs that read "Thanks Jim" and chanting "Jim, Jim, Jim."

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

Jeffords's shift — described as a "New England trick" by an incensed Oklahoma Republican, Sen. Don Nickles — will immediately increase the clout of his six-state region and the Democrats who represent it: Senators John Kerry and Edward Kennedy of Massachusetts, Chris Dodd and Joe Lieberman of Connecticut, Jack Reed of Rhode Island and Patrick Leahy of Vermont will all gain newfound power.

Leahy, Jeffords and Kennedy are slated to become chairmen of major committees, Kennedy at Education, Labor and Pensions, Leahy at either Agriculture or Judiciary and Jeffords at Environment and Public Works, a committee he has always coveted but which increasingly conservative Republicans kept from him.

While political experts say, and a series of interviews in the state yesterday show, most Vermonters support Jeffords — he has been in the House or Senate since 1975 and won reelection with 66 percent of the vote last November — there was some strenuous opposition from state Republican Party leaders, old political allies and from some corners of the business community.

"I feel like I've been punched in the stomach," said James Johnston, a Montpelier undertaker and long-time Jeffords political confidant and fundraiser. "I think this does a real disservice to all the colleagues he has served with. He's made 49 Republican enemies."

Rudolph "Skip" Vallee, the state's GOP national committeeman, said Jeffords should have done the "honorable thing": resign his seat and run as an independent in a special election.

"This is not in the interest of Vermont. We have a Republican president, and this isn't going to help us get things for our state," Vallee said.

And Charles Handy, a Burlington business owner and real estate investor, said he considers Jeffords "a quitter." Jeffords "should have stayed in the party and fought for what he believes in, but he chickened out."

But those were minority views in a state where Jeffords was the only topic of conversation over the past two days. Most Vermonters respect him, and they were smiling yesterday as the man known for his off-the-rack suits and aw-shucks personality told the Senate conservatives from the South and President Bush to get lost.

In the debate over Jeffords's motives — pillar of conscience or rank opportunist — most people interviewed yesterday on the streets of Burlington, the overgrown college town that is the state's largest city and a liberal bastion, gave Jeffords enthusiastic support.

The state's two largest daily newspapers, the Burlington Free Press and the Rutland Herald, published supportive editorials.

"I am proud to be a Vermonter today," said David Lines, sitting at the counter of the Oasis Diner, where he is the manager. "Vermont has a long tradition of independence, and what Jeffords has done has made us proud."

Mark Kaplan is a Burlington lawyer and former Democratic state chairman who ran against Jeffords for a House seat in 1982. Kaplan got only 27 percent of the vote, but he could muster nothing but plaudits for Jeffords yesterday.

"He is a very nice guy, a real gentleman," said Kaplan. "And he does represent Vermont very well, culturally and on the issues."

Jeffords, whose mentor was the late Sen. John H. Chafee, has long focused mostly on education, the environment and protecting his state's beleaguered dairy farmers. If there is a quintessential Yankee New Englander in the Senate, it is Jeffords, a Congregationalist, Navy veteran, and graduate of Yale and Harvard Law School.

Now 67, he has deep roots in Vermont, where his father was chief justice of the state Supreme Court.

Elected to the state Senate at age 32, Jeffords tangled with the conservative wing of his party, losing a GOP primary for governor in 1972. He bounced back, winning his 1974 House race. He has been in Washington since.

Jeffords and his wife, Elizabeth, married in 1961, had two children, and divorced in 1979. Seven years later, the couple remarried.

Over the years, Jeffords has carved a record that is everything President Bush and the new breed of Southern conservatives who run the Congress are not.

He is sometimes quixotic, such as in his many unsuccessful attempts to win approval for a national bottle deposit law and his attempt to outlaw job discrimination against gays.

Jeffords supports legalized abortions, federal aid to public schools, especially for special-education programs, and strong environmental laws. When he was in the House in 1981, Jeffords was the only Republican to oppose President Ronald Reagan's tax cut.

The last straw, say some who know Jeffords well, came when he received signals from Republican leaders and the White House that one of his pet projects — a dairy farm subsidy program called the Northeast Dairy Compact — would be killed.

Farming is no longer Vermont's major industry. Dairy farming has been in steady decline for decades; in 1964, there were 7,000 dairy farms in Vermont, today there are less than 2,000.

A local joke has it that nowadays there are more Woody Jackson Holsteins screen-printed on Ben & Jerry's T-Shirts than there are actual cows chewing their cud in the state's pastures.

But farming looms large in Vermont's psyche — many native Vermonters who work in cities are just a generation or so off the family farm. More-recently-arrived Vermonters often see themselves as residents of an agricultural Eden.

Vermont's farms may not be as productive as they once were, but they preserve the postcard meadow-and-mountain views that lure visitors and stoke the tourist business.

And Vermonters know that the demise of more farms means more development, an epithet in a state with the nation's toughest zoning and land-use laws.

"If the dairy compact goes down, that will lead to more farms going out of business," says Pat Hicks, of Shelburne, a Burlington waiter. "Pretty soon, you are subdividing the land, and then there's development."

Vermont's civil political culture is light years from the hardball election jousts and ethnic and racial scrums of Rhode Island and Massachusetts. Vermont is one of the two whitest states in the country (Maine is the other), according to Census 2000 figures.

It resembles a small, hilly Scandinavian country; the constituency of cross-country skiers is larger than any racial or ethnic minority.

A Bush administration snub of Jeffords — not inviting Jeffords to a White House ceremony honoring a Vermont resident named Teacher of the Year — has also struck a nerve in Vermont.

"I don't know why they did that to him," said Linda Robinson, of Colchester, between spoonfuls of oatmeal at the Oasis. "That seemed petty to a lot of people."

The irony, said Brown University political scientist Darrell West, is that it probably would not have taken much to keep Jeffords in the GOP fold — a few million dollars for his farm program, more money for education, and treating him with a modicum of respect in education and budget negotiations.

But, said West, the Southerners who run Congress and the White House "don't understand and don't care about New England, which is pretty foolish because when the Senate is 50-50, you better care about each vote."

Wendy Schiller, another Brown University political scientist, said the Republicans are going to get paid back through the increased clout of such headaches to conservatives as Kennedy. "Ted Kennedy is going to be more energized than he has been in years," she said.

The most intriguing question is whether Jeffords's defection is merely a quirky senator's shift or the start of a historic realignment of the two major parties, with conservative Democrats becoming Republicans and moderate Republicans becoming Democrats.

"I think Jeffords is the first wedge," said Garrison Nelson, a University of Vermont political scientist who knows Jeffords well. "There will be pressure from both sides."

One of the most obvious pressure points, Nelson says, will be Rhode Island Republican Lincoln Chafee, who says he is staying put for now, but who is close to Jeffords and increasingly upset at the conservative GOP agenda.