The bold-faced headline in the full-page advertisement in Vermont newspapers had a familiar ring: "Where DOES Pat Leahy stand?"

To Democrats, the attack on their senator set off alarm bells: Its message was identical to the "Where DOES Jim Hunt stand. . . " television ads that were credited with damaging North Carolina Gov. James B. Hunt's 1984 unsuccessful effort to unseat Sen. Jesse Helms (R).

While the Green Mountain state and the Tar Heel state are many miles apart, Vermont's battle of two political giants, Richard A. Snelling versus Patrick J. Leahy, a popular former governor versus a respected senator, is inviting comparisons to the Helms-Hunt struggle, renowned for its vituperation.

The Vermont race is critical to control of the Senate where the GOP holds a 53-to-47 majority. President Reagan publicly pressured Snelling to enter the race although the former governor had been one of his most vocal GOP critics.

Last week, trailing by more than 20 points in the polls, Snelling hit the airwaves with commercials charging Leahy with being "one of the biggest spenders in the Senate." Leahy, the ads declare, "voted to add $973 billion to the federal deficit, and he voted to increase the Senate budget committee's budget proposals by $108 billion in the past five years."

The commercials, designed by Washington consultant Charles Black who orchestrated the Helms campaign, are the first in a series of planned attacks on Leahy's record on spending, defense, trade, U.S.-Soviet relations and his role as a senator.

Already, both sides have said they expect to spend more than $1 million, making it the most expensive campaign ever in this largely rural state of 535,000.

Leahy predicts that Snelling's "distortions" of his record will backfire. "If he does two months of negative ads, I'm going to have the biggest landslide in Vermont history," Leahy said.

Snelling says his approach is not negative. "I'm going to address Leahy's record," he said. "Not him as an individual. I like him. I think most Vermonters do."

But GOP polls show that conservatives think Leahy is conservative and liberals think he is liberal. "People have a right to know what he is," Snelling said. "By any yardstick he comes out a classic liberal."

Vermont has traditionally been one of the most Republican states in the nation -- it even voted against President Franklin D. Roosevelt in 1936 -- but an influx of newcomers since the late 1960s has changed its politics.

Leahy is the only Democrat ever elected to the Senate from Vermont, but the Democrats won the governorship in 1984, albeit narrowly, as well as control of the state Senate. In a state where the taciturn Yankee is predominant, this campaign may provide a new case study on the effectiveness of negative advertising.

While the two opponents may be Vermont's best vote-getters, their personalities and backgrounds are quite different. The stocky, curly haired Snelling, 59, is a Harvard-educated millionaire businessman who formed a ski-pole manufacturing company before going into politics, serving as state House majority leader and governor from 1978 to 1984. Articulate and combative, he has a sour relationship with the Vermont news media and with many legislators, who nicknamed him "King Richard."

"Humility is not his strong suit," one state senator said.

Leahy, 46, a young Burlington prosecutor when elected to the Senate 12 years ago, is a friendly and media-conscious politician who charms audiences with his down-home anecdotes, and does not hesitate to remind them that he, unlike the Pennsylvania-born Snelling, is a native Vermonter. His ads feature him, tall and white-haired, walking through snowy paths and talking earnestly about the environment and arms control.

While each vows to campaign on a high plane, recent interviews revealed hints of nastiness to come.

Leahy on Snelling: "He likes limousines and guys with earplugs. He enjoyed the National Governors' conferences . . . . He flatters easily. He likes to be on a first-name basis with well-known figures. He'll say, 'My friend Peter Domenici.' Have you ever heard anyone call Pete Domenici Peter?

"I don't think Dick is one to have too many close friends. He's extremely partisan . . . . He finds it uncomfortable to share credit for things."

Snelling on Leahy: "He is the fourth-heaviest user of the franking privilege, on a per-capita basis, of all the U.S. Senate. In three months in 1985, he mailed 594,000 pieces of mail in Vermont to 150,000 families in the state . . . . His people bragged about the targeting system he has. If people are in the National Guard they get mailed pieces about wonderful things he's done for the National Guard. If they're farmers they get mailings saying how he's helped the farmers. I have a chart showing Leahy's mailings with the peaks and valleys. The peaks are clearly identifiable with elections . . . . It's not surprising that incumbents would use the tools available to them. It's not immoral . . . . But if they are not senators for life, someone among the populace has to find out if the impressions created are correct."

Leahy's deft use of incumbency was obvious on a recent two-day swing through the state. In Brattleboro, he reminded supporters in this dairy state that he is the only northeastern senator and ranking Democrat on the Agriculture Committee. He boasted of his ranking spot on the Appropriations subcommittee that oversees the Environmental Protection Agency. He got EPA Administrator Lee M. Thomas to climb a Vermont mountain devastated by acid rain.

Talking to a senior citizens' convention in Montpelier, Leahy laid out his plans to introduce a "Better Health Care Bill" revising unpopular Medicare regulations. He brought a caseworker to help with constituent complaints.

Snelling, speaking to the same group a few hours later, was short on specifics. "Although I propose no new programs, I propose we work much harder at evaluating the programs we do have." Block grants from Washington are preferable to targeted funds, he said, because states design and operate programs better than the federal government.

As head of the National Governors' Association, Snelling favored tax increases and a slowdown in military spending to reduce the deficit, suggesting at one point that Reagan's domestic spending cuts were leading to an economic "Bay of Pigs."

With this moderate-to-liberal background many Democrats think Snelling will have a hard time making his "big spender" attack against Leahy stick. Leahy voted for the Gramm-Rudman-Hollings balanced-budget act and is quick to point out that Snelling left the state a deficit of $35.8 million by undercalculating effects of the Reagan tax cut. (Vermont's income tax is tied to the federal tax system.)

Leahy aired a quick counterpunching ad this week, showing him eating in a diner. "Leahy is a budget balancer all right," it said. "For every new dollar he voted to spend, he voted a $1.10 cut . . . . For the last four years, Patrick Leahy has voted to spend even less than President Reagan asked for. Big spender? The man is cheap."

For those who try to call the election this early, there are caveats on both sides. Vermont has never rejected a senator seeking reelection. On the other hand, Leahy has won both his elections with less than a majority, scraping by a relatively unknown challenger in 1980 by a mere 2,755 votes