Look for Walter F. Mondale to come out smoking at Sunday night's debate in Louisville.

Look for him to try to frighten voters about a second Reagan term, belabor the president for hiding behind a happy-talk campaign, pounce on any presidential misstatements and set a fiesty tone right from the outset.

And don't be surprised if, even against a popular incumbent and consummate television performer, the underdog "wins" the debate.

So goes the preview from Mondale advisers and Democratic strategists, who say they think their long-shot candidate goes into Sunday's crucial showdown with the perverse advantages of having nothing to lose and no lofty expectations to meet.

"This is one of the few events of the campaign that I'm actually looking forward to as a Democrat," said Robert Squier, a media consultant who is not working for Mondale.

"It's Mondale's best format," he said. "Everybody is fond of pointing out defects in his telegenic qualities. But he's best in debates, when he forgets about trying to modulate his voice and digs right into the issues. There's animation and energy and a clear sense that he knows what he's talking about."

Mondale advisers will reveal little about their specific tactics for the 90-minute encounter, other than to promise, in the words of campaign chairman James A. Johnson, that Mondale would be a "a very aggressive" debater stepping up to "the opportunity of a lifetime."

But on one point all agree: It's too late, and they are too low in the polls, to fret over the perils of coming on too strong against a genial, aw-shucks incumbent.

"The voters want Mondale to point out the differences in vivid terms," said Greg Schneiders, a former aide to President Jimmy Carter and Sen. John Glenn (D-Ohio). "As long as you don't come off as whiny, mean-spirited or unremittingly hostile, then I don't think there's much risk in going after a sitting president. People overstate that risk."

But there are risks in relentless attack. The biggest, Mondale's advisers concede, is that to some extent it might reinforce Reagan's case that the choice this year is between optimism and pessimism.

For example, Johnson told reporters in a briefing this week, "Reagan's America is a perfect country, only waiting for its next pep rally. We have an opportunity [in the debate] to say that Ronald Reagan is so out of touch that he doesn't see toxic waste . . . , hunger, malnutrition and pain and suffering in America."

Overkill? "The truth is that people are feeling pretty good about their country right now," Schneiders said. "You don't want to come off like the skunk at the tea party."

"We won't be pessimistic," said campaign manager Robert G. Beckel. "What we'll do is point out all the areas where we can do a better job."

Beckel says he believes that the biggest payoff will be the simple act of "engaging" the interest of an expected 80 million-plus viewers in the presidential campaign. "I don't think they [the voters] have been paying as much attention during September as I thought they would," he said.

On the ticklish question of whether to be content with attacking Reagan's policies, or to go beyond that and criticize the way Reagan handles the office, Beckel hinted yesterday at a two-track strategy.

The frontal attacks will be against the president's record. But Mondale will also be primed to jump on Reagan when he gives an answer Mondale perceives as either incorrect or unresponsive.This way he'll try to make the subliminal point that Reagan does not have full command of his office.

By standing up to Reagan in this manner, strategists hope, Mondale will display a forcefulness of character that, according to the polls, voters have not yet found in him.

"He needs to say things like, 'Hey, wait a minute, Mr. Reagan; you didn't answer that question,' " said Neal Oxman, a Philadelphia-based Democratic media consultant. "He needs to talk directly to the guy and let people see he's not afraid to mix it up."

Mondale said yesterday that he would "live by the rules," which call for no direct questioning of one candidate by the other. Still, there are many ways to confront an opponent without posing a direct question.

Squier advised: "The first thing to remember in a debate is that once you're on stage, everything that's been negotiated is out the window. There are no rules, except the rules of fair play."

Meanwhile, Mondale advisers agreed that one of the best things Mondale has going for him is the widespread perception -- mistaken, they believe -- that he is a wooden, drab candidate.

"No question, he's helped by the low expectations," said one top Mondale adviser, "unless, of course, he lives up to them."