Ronald Reagan's base remains rock-hard in this conservative Republican town that gave him almost a 2-to-1 victory over Gerald R. Ford four years ago and now threatens an even higher margin over George Bush, his chief rival in tomorrow's presidential primary election.

Despite Reagan's 1976 primary win here, he lost the state narrowly to Ford. Thus, the hilly town of Derry seems an ideal spot to quiz Republican voters -- we interviewed 62 of them -- for an answer to this question: With Bush rising dramatically in the national polls following his Iowa victory, what is happening to 69-year-old Reagan's vaunted base? Is it holding firm or shifting to Bush?

The answer, derived with the help of Patrick Caddell's Cambridge Survey Research and four interviewers headed by field supervisor Connie Zimmerman, is: yes, it is holding; no, it is not yet shifting to Bush. That verdict preceded the raucus Saturday night encounter in Nashua, which might further enhance Reagan's strength here.

"Bush is just too damn smooth," a fast-talking, 45-year-old engineering draftsman told us. "He's a little like Frank Burns in "M*A*S*H."

Fair or not, that sums up a measurable opinion of Bush by several of the 39 Republicans who plan to vote for Reagan tomorrow. Twelve of our 62 voters want to vote for Bush, four for Sen. Howard Baker and three for Rep. John Anderson. The rest are undecided.

"Bush had a kind of phony appearance in the debate," said a 43-year-old housewife who doubles as a federal meat inspector and was one of the 15 voters who took the trouble to watch the Feb. 20 all-candidate debate. That debate clearly hurt Bush in this Reaganite stronghold.

We asked whether voters felt that Reagan -- or Bush -- had helped or hurt himself in the debate. Those favorably impressed by Reagan outnumbered those favorably impressed by Bush 2-to-1, while Bush was felt to have hurt himself in the debate for more than Reagan.

Other evidence of Reagan's concrete-reinforced base, a legacy of over 15 years as the cutting edge of Republican conservatism, came in his 80 percent favorable rating. Bush's rating among these Republicans was an anemic 50 percent -- far below his score among Democrats in our interviewing earlier this month in liberal Nashua.

As viewed here, Bush has not yet established his political credentials and seems to be skating on quarter-inch ice. A 52-year-old consulting engineer, who told us he had just stripped off his Bush bumper sticker the day we interviewed him, tried to explain why: "I've changed to Jerry Ford because I don't think any other Republican can get the Democrats out of the White House," he told us, his wife (who backs Baker) nodding energetically. "I admire Bush and his experience, especially in the CIA, but his campaign has just seemed to run down."

Our interviews suggested an extraordinary intensity in the way these Republicans indentify themselves with Reagan on major issues. Asked which candidate, Bush or Reagan, seems better able to handle the Soviets, U.S. defense problems and the economy, Reagan won almost unanimously among those who had voted for him in 1976 over Ford. But he also won -- though by much closer margins -- among those who backed Ford did not vote at all in 1976.

Even here in Reagan country, the former California governor perceptibly suffers from the age factor. The sole 1976 Reagan voter planning to defect to Bush this year told us age was the reason; almost half of the 1976 Ford voters now say that Reagan is "simply too old" for the presidency. One middle-aged woman, a U.S. postal inspector, told us she was reluctantly leaving Reagan even though "I sure hate to hold age against him." Was she moving to Bush? Not yet, she said. "I don't know him well enough."

Indeed, fully 22 of our voters agreed with the statement that "I don't know George Bush well enough yet to support him for president." For those who disagreed, however, one of Bush's brightest credentials here is his foreign policy experience, particularly his 13 months running the CIA. "With his CIA background he must know a lot about foreign problems," a 50-year-old life insurance salesman said. "That's what Carter lacked when he came up from Georgia and what Reagan never got in California."

But that perceptionof Bush is still fuzzy in this Reagan heartland in Southern New Hampshire, as is Bush himself. Coupled with the evidence of Reagan's solid base, this suggests a nip-and-tuck race tomorrow, particularly if the sentiments in Derry reflect anything like the state as a whole. It hints, too, that if his national base also holds, Reagan will be around for quite a while no matter what happens here.