When his staff says that Democratic nominee Walter F. Mondale "gained momentum" from his "victory" over President Reagan in Sunday night's televised presidential debate, it sounds like an empty sports-page cliche.

But it was no cliche for Steve Elmendorf, the campaign aide responsible for producing a crowd for Mondale's midday rally in John F. Kennedy Square here today.

"It makes an amazing difference" that Mondale did well in the debate, Elmendorf said, when the rally had been concluded successfully. "The people I call the party hackery are all pumped. The elected officials, the union leaders suddenly have gotten a lot more willing to put out the bodies and the buses."

With their cooperation, Elmendorf managed to assemble a respectable crowd of several thousand people on a damp, foggy day at a site that Democrats have largely abandoned for rallies because of the difficulty of getting people to come into downtown Detroit in the middle of a working day.

"Momentum" was no empty cliche either for Tom Cashman, press secretary for the Mondale campaign in Ohio. For a week, he had asked Cincinnati radio and television stations to broadcast the hour-long "citizens forum" with which Mondale began this campaign day.

One television station and one radio station in the Ohio River city had been persuaded to give live coverage to the speech and question-and-answer session. Then on Monday, without further effort on Cashman's part, Cincinnati's two other television stations and one more radio station decided that Mondale had become newsworthy enough to cover live.

"It was the debate that did it," Cashman said. "They realized that Mondale might be back in the race."

Political campaigns are essentially exercises in mass persuasion, and nothing is more persuasive than success. In August and September, Mondale had struggled to overcome the image of being "a loser," hung on him by speaker after speaker at the Republican National Convention and reinforced by newspaper and television reports of his struggling, sometimes inept, campaign.

For weeks, polls showed him slipping further behind President Reagan. Now, for the first time since the Democratic National Convention, Mondale has begun to change public perceptions, and the effects are tangible.

It has given Mondale a needed shot of confidence as a stump speaker. "Who won the debate?" he bellowed at the Detroit audience. "Mondale," they shouted. "Who's going to win the election?" "Mondale," they answered.

Saying it does not make it so, of course, but no campaign can rise above the level of its backers' enthusiasm. Part of the momentum Mondale gained from Louisville is a lift in the spirits of his supporters.

Steve Stevens, who had been spending about three hours an evening as a volunteer at the Mondale telephone bank in Cincinnati, said, "We'd been phoning Democrats for four weeks, asking if there were any unregistered people in their households, and they had been polite when we said we were calling for Mondale. But last night -- last night, it was different. They wanted to talk about how good he'd been and how important the election was."

Rep. Barbara B. Kennelly (D-Conn.) said in a telephone interview that she had noticed the same change in her Hartford district. "On Sunday, when I mentioned Mondale's name to people I met campaigning, they didn't want to hear it. On Monday, they were ready to listen . . . . We've got our foot in the door."

In New Jersey, the Mondale campaign press secretary, Emma Byrne, said that "aside from what the debate did in giving the general public a clearer perception of the issues, it just re-energized the staff and volunteers. That's probably as significant as anything."

How long the upbeat mood can be sustained depends on Mondale and on outside forces. Today's crowds were less ebullient than those in New York on Monday. The candidate and his aides are hoping the next public polls will confirm what they say are their own findings of "significant gains" in Mondale's favorable ratings. They hope running mate Geraldine A. Ferraro's performance in the television debate with Vice President Bush on Thursday will be another plus.

Meantime, Mondale carries the title of "the Louisville Slugger," awarded him in Cincinnati by Ohio Gov. Richard F. Celeste -- along with a baseball bat with that label. He never had that before.