Boston — Nothing is more changeable than the New England political scene -- except perhaps for the region's weather. But this election-year so far has been a virtual whirlwind of uncertainty and surprise.
In the past nine months:
* In Vermont, would-be successors were lining up after Gov. Richard A. Snelling, chairman of the National Governors' Association, announced last fall that he wouldn't seek reelection. Now, the three-term state chief executive has not only reversed himself, he is headed for a free ride to the GOP nomination.
* Massachusetts Lt. Gov. Thomas P. O'Neill III -- son of the speaker of the US House -- had been aggressively campaigning for governor. He has since abandoned the race and is heading for at least temporary political retirement at year's end.
* And archconservative Meldrim Thomson Jr., the former three-term New Hampshire governor who once praised South Africa's treatment of blacks and lowered the flag to half-staff when the Panama Canal treaties were signed, had announced last winter that he wouldn't seek election. But the controversial Republican has at least partially reversed himself and states he may yet launch a campaign for governor as an independent.
In this six-state region -- the smallest and most liberal section of the United States -- the conservative political tide of the 1980 election may face one of its stiffest challenges.
Although President Reagan is not on the ballot, the policies of his administration, including increased defense spending and reductions in funding levels for other programs, seem more and more likely to have a major impact on this fall's balloting here. Campaign issues vary from state to state, but candidates' attention in most of New England, especially in gubernatorial contests, is directed toward combating unemployment, a problem of major proportions in much of the region.
Still, Republican incumbents, less than fully supportive of Mr. Reagan and his policies, are especially under attack from the GOP far right in Connecticut and Vermont.
Among the hundreds of elective offices at stake on next fall's ballots are all of the region's governorships, five seats in the US Senate, and all 24 US House seats to which the six states henceforth will be entitled.
A state-by-state analysis follows: Connecticut
The focal point of the Connecticut campaign is a tussle for the US Senate between reelection-bent Republican Lowell P. Weicker Jr. and four-term Democratic US Rep. Toby Moffett.
The latter won his party's endorsement at the state Democratic convention July 16 and 17. More in doubt had been the outcome of the Republican convention the following week, in which Senator Weicker, the GOP liberal and 12-year incumbent, was challenged for endorsement by Prescott Bush Jr., a conservative and brother of Vice-President George Bush.
Although unsuccessful in wresting the party activist backing from the senator , the challenger in the process polled 35 percent of the delegate vote and thus qualified to press his candidacy for the nomination in a Sept. 7 primary.
In the interest of party unity, however, Mr. Bush decided against such a course, which almost certainly would have split Connecticut Republicans, perhaps ultimately costing the GOP the Senate seat. The Greenwich conservative now is supporting the Weicker third-term bid.
A June public-opinion poll, published in the Hartford Courant, gave Bush a 56 -to-32 percent lead over the senator, but against Democrat Moffett, the vice-president's brother trailed 48-to-36 percent. In a three-way contest, involving the two Republicans and the Democrat, Senator Weicker was favored by 31 percent, Representative Moffett by 28 percent, and Mr. Bush by 24 percent.
Senator Weicker, a political maverick who has not supported the Reagan administration on some key votes, has long been a target of the National Conservative Political Action Committee.
In the gubernatorial race, Democratic Gov. William A. O'Neill is another example of the flip-flops in the New England political scene. For health reasons , Mr. O'Neill late last year had considered not seeking reelection. But he returned to his desk last winter. He has the strong backing of the state Democratic regime. His gubernatorial challenger for the party nomination is state House Speaker Ernest N. Abate.
The Republican governorship challenger on the November ballot is expected to be former state Senate minority leader Lewis B. Rome of Bloomfield, who narrowly bested Richard Bozzuto of Watertown, another former state Senate Republican floor leader, for their party's endorsement.
Mr. Bozzuto has decided not to press his quest for the nomination through a Sept. 7 GOP primary so Mr. Rome has the nomination without further challenge. Maine
Maine Democrats have renominated Gov. Joseph E. Brennan for a second four-year term. His Republican foe will be Portland attorney Charles L. Cragin, who outpolled two GOP gubernatorial rivals in the June 8 state primary. Jobs and economic development are key issues in the coming campaign. The incumbent is out to capitalize on the voter-approved bonds, initiated by his administration, to build fish piers, potato-packaging sheds, and the controversial Portland dry dock for the Bath Iron Works.
Mr. Cragin, whose career includes that of a lobbyist for medical interests and various industries, was the leader of a drive to put the question of state income-tax indexing on next November's ballot. The measure could save Maine residents some $37 million by restricting levies.
Earlier this year, it looked as if US Sen. George J. Mitchell (D) of Maine was headed for a stiff June 8 primary challenge from former Gov. Kenneth M. Curtis. As it turned out, Senator Mitchell, appointed May 1980 to take over from Edmund S. Muskie when the latter was chosen secretary of state, breezed onto next November's ballot without any intraparty opposition.
Republicans have chosen fourth-term US Rep. David S. Emery, whose present House district comprises half the state's population, to run for the former Muskie senatorial seat. Massachusetts
Former Gov. Michael S. Dukakis appears to have a fairly wide lead over incumbent Gov. Edward J. King in the battle for the Bay State's Democratic gubernatorial nomination in the Sept. 14 primary. Delegates at the party's May 22 nonbinding convention favored the liberal-leaning Dukakis 2 to 1 over his conservative successor.
Meanwhile, the Republican gubernatorial primary will pit millionaire businessman John R. Lakian, winner of the March 4 GOP state convention, against two more liberal challengers -- former Boston City Councilor John W. Sears and State Rep. Andrew H. Card Jr.
Besides the winners of the Democratic and Republican primaries, the election race also includes two political newcomers, Libertarian candidate Rebecca Shipman and Francis P. Rich, a former Democrat and adviser to Governor King who broke with the head of state last year and is running as an independent.
Democratic US Sen. Edward M. Kennedy, who like Connecticut's Weicker has been targeted for political extinction by national conservative forces, faces a challenge from another millionaire, Republican industrialist and political newcomer Raymond Shamie, who is very much the underdog.
New England's hottest battle is the US House contest under way in the newly crafted Massachusetts Fourth Congressional District. First-term Democrat Barney Frank, a liberal, and eight-term Republican Margaret M. Heckler, a conservative, have been thrown onto the same elective turf. The commonwealth is losing one of its current 12 seats in the US House. New Hampshire
Gov. Hugh Gallen should have no difficulty winning Democratic nomination to a third two-year term. His prospects for a November ballot victory, however, are uncertain and could hinge on the extent to which President Reagan becomes involved in the campaign.
Reagan, who gained his first step to the White House with a victory in New Hampshire's 1980 presidential preference primary, has several prominent GOP activists from the state, including two ex-state Republican chairman, Gerald P. Carman and Carroll Jones, in his administration.
At least five Republican gubernatorial candidates now are in the running for the nomination, including Robert Monier, the state Senate president, John Sununu , a Tufts University engineering professor, and Louis D'Allesandro, assistant to the president of New England College.
The latter was runner-up to former GOP Governor Thomson in the party's gubernatorial primary two years ago, and Mr. Sununu finished a close second behind now US Sen. -Warren Rudman in a crowded field for the 1980 Republican nomination for the Senate seat.
The ''independent'' candidacy of former Governor Thomson seems likely to remain on a political back burner until after the September primary. If the GOP nominates the more liberal Mr. D'Allesandro instead of a conservative, such as Mr. Sununu or Mr. Monier, Thomson then is expected to press his comeback, thus forcing a three-way tussle for the governorship on the November ballot.
State finances seem likely to be the key campaign issue, with Governor Gallen in his candidacy announcement calling for tax reform as the top priority but stopping short of pledging to veto any measure that might involve imposition of either a broad-based sales tax or income tax. This is the only state in the nation without at least one of these levies.
Republican candidates Monier and Sununu have ''taken the pledge,'' as it is called, promising to veto any such legislation. Thomson, too, can be expected to make such a commitment the cornerstone of his comeback effort, as it has been in each of his gubernatorial campaigns over the past decade.
This is first election since the passing last year of William Loeb, the controversial, longtime publisher of the Manchester Union Leader who was a major force for decades on the state's political scene. Rhode Island
Second-term-bent US Sen. John H. Chafee, after a free ride to the GOP nomination in the Sept. 14 primary, faces a November Democratic challenge from the winner of that party's primary, expected to be Julius Michaelson, a former state attorney general who was endorsed at the state's June 12 convention.
Vincent Marzullo, a former Democrat and former state jobs-training director under three governors, is the Republican convention endorsee for governor. He is not expected to face formidable opposition in the September primary.
Democratic Gov. J. Joseph Garrahy, seeking a fourth two-year term, has a free ride to renomination and is definitely the front-runner on the November ballot. Vermont
Republican Governor Snelling is heading for a free ride to a GOP renomination. His probable November ballot rival, Lt. Gov. Madeleine Kunin, is facing little more than token opposition for the Democratic nomination.
The governor's April veto of legislation raising the state's minimum legal drinking age from 18 to 19 has been sharply criticized by Mrs. Kunin, and it may develop as a key issue in the competition for the executive chair.
Six months ago, US Sen. Robert T. Stafford seemed all but sure of reelection, since he faced no formidable opposition from within either the GOP or the opposition party. Now the veteran Republican, a political moderate by Vermont standards, faces what could be keen competition from both liberals and conservatives.
In his own party, Senator Stafford is being pressed by conservatives John McClaughry, a former White House aide, and Stewart Ledbetter, a former state banking and insurance commissioner who two years ago narrowly missed toppling Democratic US Sen. Patrick Leahy. They charge that Stafford has lost touch with Vermont voters and has not supported the Reagan administration on some key votes.
The September GOP primary winner is expected to meet liberal Democrat James Guest, a former Vermont secretary of state, who from the political left has already begun lashing out at the senator and his 12-year record in Washington. Two fellow Democrats -- state Sen. Thomas McGregor and Earl Gardner -- are vying with Mr. Guest for their party's senatorial nomination.