Since he was 9 and campaigning door-to-door for Alf Landon, Richard A. Snelling has considered himself a true-blue conservative Republican. But suddenly, this winter, he finds himself one of the more vocal defenders of liberal social programs.

Snelling, 54, the prickly and scholarly intellectual Yankee governor of Vermont, has been one of the principal voices asserting that President Reagan's spending reductions in federal grants-in-aid to cities and states have cut too deep too fast.

In a speech to the National League of Cities earlier this month he criticized the administration's economic recovery program as a melange of conflicting policies that were leading to an "economic Bay of Pigs."

"I have wanted what the president is trying to do for the last 20 years but I'm not interested in trying to win the whole game for every war in the first quarter," he said in a recent interview.

"I was driven to being a conservative instead of a moderate by the excesses in which I grew up. I just didn't believe that New Deal Democrats were responsible," he continued, adding with heavy irony, "now I'm being driven to be a liberal, which is a political death wish. I'm driven to be a liberal by what I see as excesses of zeal in a good cause."

Normally the opinions of the governor of small and remote Vermont would not count for a whole lot in the determination of national policy but Snelling, a big and broad-shouldered man with great energy, holds the presidency of the National Governors Association.

And he has not hesitated to use the office forcefully to call for a moratorium on cuts in social programs over the next three years while cities and states absorb the reductions approved by Congress last summer.

Others have begun to say the same thing and the irony is that they are not Democrats but powerful Senate GOP allies of the president like his close friend Paul Laxalt of Nevada, Majority Leader Howard H. Baker Jr. of Tennessee and Budget Committee Chairman Pete V. Domenici of New Mexico.

Snelling has worked successfully to line up the leaders of the major organizations representing state and local governments in support of a call for "restraint" in the budget cutting. They want a domestic summit with top administration officials and congressional leaders for hard give-and-take discussions about the budget and the president's oft-stated dream of a new "federalism" with control over social programs shifted from Washington to the states and cities.

Snelling has been enthusiastic about a new federalism but not the way Reagan has defined it. He suggested in his Detroit speech that it represented an "old frontier vision" that would not return the country to the principles of the Constitution as the president has said he wishes to do, but would move the nation further back in history to the era of the unworkable Articles of Confederation.

Reagan has said he does not believe there is any national responsibility to redistribute resources between rich and poor states. Snelling disagrees emphatically.

"I have invested a great deal of my life in the Republican philosophy and the Republican philosophy has never meant to me, 'I keep mine and you worry about yours,' " he said.

For the moment, Snelling's broadsides, however infuriating, are being tolerated at the White House. The president had him in the Oval Office last week and listened to Snelling's appeal that budget and tax cuts be slowed substantially. Afterward Snelling described the session as "pleasant, harmonious" but "not encouraging to my major thesis."

Never a favorite of the administration, Snelling is remembered for his arduous efforts to stop Reagan's nomination. The Vermont governor tried to put together a "Draft Ford" movement and when that fell apart jumped onto the Howard Baker-for-President bandwagon long after it had begun to sputter and run out of gas. In the White House there is still the memory that this critic from within the president's own party climbed onto the Reagan campaign somewhat after the 11th hour.

For the time being, however, he is a man the White House cannot easily dismiss because what he is saying clearly reflects a mood out there. Not all would go along with the strong and colorful expressions he sometimes uses.

Some of his fellow GOP governors, such as James Thompson of Illinois, pointedly disassociated themselves from Snelling's "economic Bay of Pigs" remark but when the Governors Association executive committee drafted a letter to Reagan a few days after the Detroit speech in which it was made, it was Thompson who had language inserted indicating that a good and adequate road system and an educated and highly trained workforce were, like troops and weaponry, essential to the nation's security.

The contemplation of further budget cutting coincides with the strong adverse impact the recession is having on many states with Republican governors, many of whom face reelection next year.

Oregon and Washington, big timber states, are reeling from the plunge of the housing industry. The recession has only deepened the troubles of Republican governors throughout the Midwest who, long before it came, had been grappling with the problems created by the auto industry.

Snelling's Vermont has weathered the storm nicely so far, however. Unemployment is below 5 percent and Vermont's finances are in such good shape that Snelling was able to use state funds to cover losses from federal cuts in such programs as alcohol rehabilitation, Medicaid and weatherization.

The skiing industry and a boomlet in computer and electronics firms in the last two decades are the base of a health economy after generations in which Vermont, bucolic and isolated, relied mainly on dairy farming.

Snelling personally participated in the economic rebirth of Vermont. He moved there in the early 1950s and later started a prosperous company in a well-off suburb of Burlington that made cup hooks and other household hardware before expanding to manufacture ski poles and ski and bike racks.

As he built his business, he moved in and out of politics, serving three terms in the state House where he became majority leader. He ran unsuccessfully for lieutenant governor and for governor before being elected governor in 1976.

In his third two-year term now, he is praised in Vermont for the "no nonsense" way he has managed the state, but his abrupt manner and self-assurance have rubbed many the wrong way, even those who profess strong admiration for him.

"He's personally charming one-on-one. In public, he's an arrogant b------," says Steve Terry, the managing editor of the Rutland Daily Herald. "He's run government pretty lean but that's not too tough in a state with a budget of $300 million a year."

"He is very thin-skinned," said Charles Butler who ran two of Snelling's gubernatorial campaigns and served three years as his executive assistant. "He doesn't take criticism well, not even from people who consider themselves friends. He's just a hard person to get to like."

But there is much respect from them for his keen intellect and the way he throws himself full force into whatever he's involved in, whether it's running the state, piloting an airplane or one of his favorite pursuits these days: reading, rereading, analyzing and dissecting the Federalist Papers.

"He's constantly testing," said Terry. "He won't even think about the problem until he's tested the facts."

It is an intellectual approach learned as a child at the dinner table in Allentown, Pa. His father was a chemist and inventor, and like his mother, a patent attorney. Dinner table conversation consisted of the father posing a problem to his children and giving a reward to the first to come up with the right answer.

Snelling spoke of his father in an interview as "highly disciplined. That probably is what makes me the weird person that I am."

Snelling has already announced he is leaving office when his term as governor ends in January, 1983, to take time out to think and write, he said in the interview. He has been talking to the people at the Kennedy School at his alma mater, Harvard, about a job there.

But no one who knows him well believes he is retiring from politics. There is some feeling in the administration that his assaults on Reagan stem from a mistaken belief that the White House had blocked him from running for the Senate by persuading the incumbent Republican, Robert T. Stafford, not to retire.

But Snelling's former aide, Butler, says the governor of Vermont would "rather be president and I'm not joking."

"You got to understand the man. He likes to manage things. He likes to control things," Butler said. "He has no desire to be one out of 100. His desire is to be one out of one--the president of the United States."