The logic of the situation, Republicans here are quick to say, is all on their side as they set their sights on a seat in the U.S. Senate.

There's a conservative tide running. Ronald Reagan carried the state in 1980. A conservative Democrat is governor. A successful tax revolt by conservatives is behind them.

Their candidate, Raymond Shamie, is a conservative in the Reagan mold, a man of much energy and wealth, both of which he is heavily investing in his own campaign.

The logic ends there, bumping up against the fact that the incumbent is a Kennedy and this is, after all, Massachusetts, where Edward M. Kennedy has won reelection with majorities of 72, 62 and 69 percent.

"He's the best candidate we ever had against Teddy Kennedy," said GOP National Committeeman Gordon Nelson, in a burst of enthusiasm that others echo. "But it's the proverbial uphill fight--how's that for an understatement?"

Long shot that it is, the attempt to unseat Kennedy has aroused more interest than the previous fruitless assaults, partly because of Shamie's money and partly because national conservative organizations originally seemed likely to swamp the state with support to oust Mr. Liberal from the Senate.

What some envisioned as the great ideological contest of 1982 has not yet come to pass, however. The National Conservative Political Action Committee says it has invested $500,000 in an anti-Kennedy campaign, but is conducting polls to see if going deeper is worthwhile.

Shamie has sunk $700,000 of his personal fortune into the effort, but has not found as much financial help from like-minded national political committees as he'd hoped. An ambitious $5 million campaign, planned early this year, has shrunk of necessity to about $3 million.

The Republicans struggle to show that their man is on the brink of believability as a candidate who just might pull off the big one. Shamie polls taken in March and last September show a substantial increase in the number who recognize his name and a declining proportion wanting to reelect Kennedy.

Still, the smart money is staying out.

The David in this battle with Goliath is an amiable, earnest 61-year-old businessman who built a metal-bellows manufacturing firm from scratch and has turned to politics, he says, to make the country safe again for such entrepreneurship.

The solution for Shamie is more, not less, Reaganomics. Last year's personal tax cut, he thinks, should have been 35 percent instead of the 25 percent Reagan finally accepted--and not nearly enough "fat" has been cut out of the budget. He asserts that $30 billion to $50 billion more can be trimmed from subsidies to special-interest groups.

Shamie fervently endorses the Reagan defense buildup, claiming the Soviet Union has outspent the U.S. in arms for 10 years. He is against abortion, gun control and "forced" school-busing experiments, which he claims wrecked Boston's system.

The only issue on which he and Kennedy seem to be close is the nuclear weapons freeze, which lies close to the political surface here. Shamie says he favors a "verifiable, mutual, balanced and equitable" freeze on nuclear arms, language close to that of the resolution Kennedy is cosponsoring in the Senate.

Shamie contends Kennedy has come to that position lately.

"I have the impression he's for a unilateral freeze, too," he said. "That's what we did during detente, and what did the Russians do? They outspent us for 10 years."

The GOP dream here is of a coalition of traditional Republicans with blue-collar, working-class voters from Boston and the surrounding industrial suburbs, much as Reagan had two years ago.

The evidence so far is slim that Shamie can put that together, given Kennedy's demonstrated vote-pulling power. The only major example of Democratic defection was his loss in 1976 of two wards in South Boston where anger over school busing was strong.

In two rounds of television advertising and thousands of pamphlets, Shamie has criticized Kennedy as the archetype of the big-spending liberal and advocate of "forced busing." So far, he acknowledges, he has spent little effort explaining his own views, but vows to strengthen his public image this fall.

Kennedy has campaigned sporadically on weekends and vacations, and aides say he plans to make a major drive this fall. Their polls show no serious threat, although they agree Shamie has succeeded in making his name better known.

Shamie is convinced that only a fog of Kennedy public-relations gambits keeps the Massachusetts voter from understanding that their senator does not represent their conservative views.

Others think the voters know where Kennedy stands and don't care, just because he's a Kennedy.

"Kennedy lost those two wards in South Boston in 1976 but not as big as we had expected," recalls GOP Committeeman Nelson, trying to explain the frustrations of beating the family name. "There were a lot of people who were violently against busing and abortion and still voted for him. Why? He's a living legend. That's the real problem."