The overwhelmingly Democratic blue-collar voters in this Boston suburb, who gave Jimmy Carter a landslide over Gerald Ford in 1976, have not only turned against the president but have embraced the basic economic and defense policies of Ronald Reagan.

That is the inescapable conclusion from interviewing in Precinct 7-1 just 10 days before the Democratic National Convention. It was selected for us by Patrick Caddell's Cambridge Survey Research as prototypical of blue-collar precincts that heavily supported Carter in 1976 in one of his best states. Is it possible that such a stronghold in the only state won by George McGovern in 1972 could embrace the hero of the Republican right?

It is. Out of 79 registerd voters interviewed in Prescinct 7-1, this was the outcome (in votes, not percentages): Reagan, 37; Carter, 12; Rep. John Anderson, 22; Ed Clark (Libertarian), 1; would not vote, 2; undecided, 5.

If Sectetary of State Edmund S. Muskie is substituted for Carter, Reagan's margin shrinks (Reagan 30, Muskie 26, Anderson 19). But that is still a shockingly poor Democratic performance among voters that include only 12 avowed Republicans. Nor can Anderson's independent candidacy be blamed; more of the 22 Anderson voters prefer Reagan to Carter in a two-man race.

Typical is the 47-year-old wife of a telephone lineman, a Democrat who backed Carter last time but prefers Reagan this time. "My problem is the cost of living," she told us. "Carter just hasn't done anything about it."

The 79 voters interviewed were even more pro-Carter in 1976 (69 percent) than the precinct at large of (64 percent). When asked how they voted then, many voluntarily added, "Sorry to say," after saying Carter's name. The reason given most frequently by Carter switchers for supporting Reagan was voiced by a 40-year-old nurse: "Just to get rid of Carter."

These voters give Carter a favorability rating of only 21 percent (compared with 67 percent for Reagan, 63 percent for Muskie, 53 percent for Sen. Edward Kennedy and 46 percent for Anderson). But the upheaval in Precinct 7-1 cannot be explained by Carterphobia that will dissipate once the Carter campaign paints Reagan as a missile-rattling warmonger.

That was shown by answers to questions prepared for us by Caddell: 3-to-1 supporting higher defense spending; 3-to-1 favoring the Republican platform plank calling for "military superiority"over the Russians: 4-to-1 disagreement with the proposition that Reagan as president would bring nuclear war closer; 5-to-1 preference for Reagan over Carter in dealing with the Russians.

But it is not their sense of a diminished America in the world that most bothers these middle-income Americans. Unable to make ends meet while learning a higher salary than they ever dreamed possible (the precinct's median annual income: $19,500), these voters rate Reagan as poorest of the three candidates in ability to quickly end the recession (Anderson is, curiously, the highest by nearly 4 to 1). But it is not recession that worries precinct 7-1 today. Asked to name the nation's two biggest problems, 47 voters mentioned inflation ("the econemy" was second with 20 mentions; only 7 named unemployment). But 6 to 1, these voters rate Reagan as more capable than Carter on inflation. By 4 to 1, they prefer him on taxation.

This contradicts Democratic certainty that Reagan is on the wrong political track in calling for an immediate 10 pecent "inflationary" tax cut. The opinion of these voters on Reagan's tax proposal: 67 favor, 8 oppose, 4 don't know. Their reasoning is simple, as expressed by a 51-year-old machinist switching from Carter to Reagan: "I sure could use the money."

Some voters who agree with reagan's defense and tax positions oppose him anyway. A 35-year-old teacher told us, "I'm afraid of Reagan's kind of conservatism" and will support Anderson even while endorsing the tax cut and military superiority.

A few voters called Reagan too old, and some insisted that Carter be given another chance. But no avowed Reagan voters seemed receptive to the anti-tax cut, antimilitary superiority campaign planned by the president once he emerges from Madison Square Garden. That suggests that what is happening in Precinct 7-1 is not just another swing by supposedly volatile voters, but a basic revolution in American political attitudes.