President Reagan's grip on the electorate loosened but was not quite lost during Sunday's presidential debate, as his middle-class supporters in Mike and Sue Talbot's family room discovered new doubts about their leader and new virtues in his opponent.

The badly needed gains Democratic presidential nominee Walter F. Mondale got from the 15 middle-class voters who watched the debate in this Chicago suburb were not the sort that would show up in a quick post-debate poll. No votes changed -- but a lot of opinions shifted.

This was not a scientific sample of the country, but the differences in this group's comments before and after Sunday's debate are revealing. People who a week ago were unequivocally for Reagan said after Sunday's debate that they were probably still for him, but were troubled by some of the things he said and the sometimes-faltering way in which he said them.

They voiced concerns for the first time about Reagan's age, his intentions toward trimming Social Security and Medicare, his ties to the Christian fundamentalist far right, his desire to outlaw abortion, his hope -- according to Mondale -- of packing the Supreme Court with Moral Majority-approved judges and the damage his record budget deficits might do to future generations.

A week ago, The Washington Post had shown this group a videotape of five days of television campaign news reports and commercials. Republicans and a number of Democrats in the group had come away impressed by Reagan's performance in the campaign and the economic gains he achieved.

Those economic gains are still paramount in the minds of the pro-Reagan people in the group, but the now-sizable list of other concerns led them Sunday night at least to reconsider their early decision.

In a trial balloting just after the debate ended, and in a lengthy round-table discussion that followed, it was clear that those who had once leaned toward Mondale now had no doubts about their support of him, and those who once supported Reagan now were doing so despite new doubts.

"It was apparent that he was not as confident or sure of himself in this particular debate," said Sanford Johnson, a Republican who works in marketing for a chemical firm and had come to the debate with no doubt that he would vote for Reagan. "That raises the question that maybe he is too old and his mind isn't as sharp as it used to be."

Johnson said Reagan's debate performance caused him to shift. He is now only leaning toward Reagan and wants to watch him very carefully because he does not want a president who is not up to the job.

Adam Silverstein, a retired senior citizen, said, "I think Reagan is past his peak of his efficiency; I know that because I'm pretty close to the point he's at." Silverstein said he would feel that way even if he were not a pro-Mondale Democrat. "I didn't expect him to be as hesitant, seemingly bereft of all direction there for a few times," he said. "I never expected that."

Around the room, people nodded agreement. "I noticed that too," said Monty Clark, a schoolteacher who is a moderate-to-conservative Democrat and who has been supporting Reagan. "It was much more pronounced tonight. When he Reagan didn't have a script in his mind, he was in much more difficulty. Mondale looked like the standout tonight."

Clark typifies the problem Mondale has been having this year. His political demographics say Mondale should have his vote by now. He is a teacher and former president of the local teachers' federation, whose national organization has endorsed Mondale.

His political emotions show the same support. When Mondale finished his closing statement Sunday -- a ringing call for Americans not to mortgage their children's future with today's policies on deficits and the environoment -- Clark, a normally undemonstrative man, pumped his right fist in the air in approval. Then he marked his sample ballot for Reagan.

"Mondale's final analysis -- his summation -- was excellent . . . . I'm a Democrat and it touched a chord in my heart," Clark said. "But I don't think I can afford it right now. I don't think the nation can afford it right now. Four more years from now, I think we can afford it."

Others, such as Sue Talbot, a housewife, countered that "a certain amount of people are better off, but they the Reagan people never address the issue of all of the people who are suffering, and the people in our bracket who have been laid off."

Sanford Johnson's wife, Dian, moved in the same direction as her husband, but went a notch further to the left. She is a professional artist and a political independent who went from leaning toward Mondale to falling solidly behind him.

She liked the fact that Mondale "had a chance to put Reagan down and didn't" when, during the debate, the Democratic challenger praised the president for the spirit he has instilled in America. And she didn't like Reagan's penchant for ignoring "the figures that aren't working in his way."

Morrie Oldham, an accountant who is an independent leaning toward Reagan, was troubled by Reagan's response on Social Security.

"Reagan made a statement that he will never cut Social Security benefits to those currently receiving them," he said. "I heard currently."

Others nodded. They heard that, too, they said. And they agreed with Oldham's elaboration: "He's getting at 'Don't pin me down to what I'm going to do to people who aren't receiving them benefits yet.' "

They also are concerned about Reagan's plans for Medicare. Mondale drove that point home effectively when he pounced on Reagan's effort to recycle his "There you go again" chestnut from the 1980 Reagan-Carter debate. Mondale noted that Reagan had used that phrase in 1980 in response to Carter's suggestion that Reagan would try to cut Medicare -- and that as president, Reagan had proposed cuts in Medicare.

"Reagan gave a smart-aleck, pat answer that he had already prepared in his head -- and it didn't go down," said Bud Cherry, who runs a brake parts business and is a Republican for Reagan.

Schoolteacher and Democrat Clark, accountant and independent Oldham, and automotive businessman and Republican Cherry are three Reagan supporters who shared similar doubts about their choice after watching the debate. They had doubts about Reagan's ties to the Rev. Jerry Falwell of the Moral Majority and his social agenda, and about the prospect of a Falwell-backed collection of Supreme Court judges.

"Falwell scares me," said Oldham. "If I change to Mondale, it's going to be because of the social issues." Said Cherry: "I have the same . . . concerns -- exactly."

Cherry added that Mondale's closing statement has made him rethink his support of Reagan.

"I can look at Reagan and say that these last four years have been very good, and if I hang in there, these next four years will be very good. But these eight years may be awfully expensive in my lifetime," said Cherry. "Mondale raised a big question for me: When we enjoyed the last four years under Reagan and the next four years, are our children going to pay for it? . . . Is everything right now more important than what might be?"