Self-inflicted wounds in the last 30 days have undermined Ronald Reagan's towering lead over Jimmy Carter in this blue-collar Democratic stronghold, and there are signs of further deterioration.

A 38-year-old nurse at Waltham Hospital summed it up: "Reagan keeps opening his mouth and putting his foot in it. He's said things the past few weeks that have hurt not only the U.S. but the whole world. He hasn't thought things through."

What makes that criticism especially damaging to Reagan is that this same voter, a registered independent, told us on our visit here just before the Democratic National Convention that she wanted "to get rid of Carter," whom she backed in 1976. She had decided to vote for Reagan. "I've changed, she told us without noticeable enthusiasm.

Armed with a questionnaire prepared by Patrick Caddell's Cambridge Survey Research and aided by Mary Cappello and three ace Caddell interviewers, we questioned 73 registered voters, and found Reagan's support far down from our earlier visit to Precinct 7-1. Our totals in actual votes, not percentages: Reagan 28 (down from 37): Carter 24 (up from 12); John Anderson 12 (down from 22); don't know 5 (same) and "won't vote" 4 (up from 2). In 1976, these voters went for Carter by just over 60 percent, only slightly less than Precinct 7-1.

Interviewed at their front doors in this pleasant community (average mean income: $19,500), our voters included many we talked to a month ago. Although Reagan retains a lead, defections were dramatic, raising new questions about Reagan's ability to hold inflation-plagued, blue-collar Democrats and independants essential to victory on Nov. 4.

More attrition lies ahead unless Reagan abruptly stops saying foolish things. A middle-aged housewife who still plans to vote for Reagan made that clear when she told us that Reagan "is flippy with his mouth and he changes his mind too much." A registered Democrat, she is wavering.

At least 10 of our 28 Reagan voters expressed some alarm over Reagan's hipshooting statements about China, Vietnam and the Ku Klux Klan. But their criticisms were pallid compared with the glee of Carter and Anderson voters.

"He is too confused to know what direction he's going in and he's always rescinding," said a 59-year-old vocational teacher. "Forget Reagan. His speeches are a lot of baloney," said a retired firefighter. Both back Carter. A Colby College senior supporting Anderson told us: "Reagan makes one blunder after another. Now it's the Ku Klux Klan."

Carter has done little to help himself other than punching at Reagan's self-inflected wounds and questioining his reliability. Carter's favorable rating remains low (34 percent) and, with a single exception, he continues to lag far behind Reagan on dealing with the big economic and foreign policy issues that we ask about.

Our voters favor Reagan over Carter on ability to handle the defense issue 9 to 1; on the Soviets, 3 to 3; on the "high tax" issue, better than 2 to 1. Reagan's call for an immediate 10 percent tax cut was favored almost 3 to 1.

The Reagan problem is not issued but shooting from the hip and the appearance of confusion over his positions. We asked whether Reagan's recent China altercation had made our voters "less confident" about Reagan's ability to handle tough foreign questions. Among Reagan voters, six said they now felt "somewhat less confident"; among Carter and Anderson partisans, the results were worse. Sixteen voters said his handling of the China dispute left them "much less confident"; 12 said they felt "somewhat less confident."

Those concerns showed up in another area of Reagan's vulnerability, the perception assiduously pressed by Carter that Reagan's nuclear arms proposals could threaten nuclear war. Asked whether they agreed or disagreed that electing Reagan president would "increase the chance of nuclear war," 26 voters (including four Reagan voters) said they agreed.

By appearing confused and contradictory in some of the things he has been saying, Reagan has been unable to get out from under Carter's wholly unprovable charge. It is still early in the campaign, but voter perceptions that held such high promise for Reagan here just one month ago appear to be moving toward Carter -- not because of any new confidence in the president, for we found none, but because Reagan has been acting the fool.