A Walter F. Mondale whom aides describe as "crackling with anticipation" approaches the biggest event of his political career as an underdog candidate riding a wave of personal vindication.

"This is the debate he's wanted for more than a year," his press secretary, Maxine Isaacs, said of tonight's showdown with President Reagan on foreign policy. "He feels like a prosecutor with an ironclad case."

Mondale's exhiliration comes not just from a sure-handed sense of his material, aides say, but from an immense satisfaction at finally having broken free of the "wimp factor" that dogged him and clouded his message for so much of the fall campaign.

The Democratic nominee laid that perception to rest for most Amercians with a tough but gracious performance in the Louisville debate.

Having done so, aides say he believes that he can now get a much fuller hearing in the second nationally televised debate tonight in Kansas City, and for the remainder of the campaign.

Mondale's aides privately concede that even if he performs as well as they expect him to, the real key will be whether Reagan can bounce back from his stumbling performance of two weeks ago.

That amounts to a tacit admission that with just 16 days until election day, this campaign is stuck exactly where it began so many months ago. It is still Reagan's to lose.

Even so, there has been a bouyancy to the post-Louisville Mondale -- and a palpable sense of relief that, whatever happens Nov. 6, he will at least not have come to the end of his grueling, two-year journey never having persuaded the public that he has the personal mettle to be taken seriously as a presidential candidate.

"Looking back on it now, that period before the first debate was very tough on Mondale, maybe the toughest of his life," Isaacs said.

"He'd read the papers every morning and somebody else would be describing him as a wimp," Isaacs said.

"He was being villified for politicial reasons," she said, "yet it was somehow catching on as fact, and it seemed like there was nothing we could do about it.

"In a lot of ways, it was tougher than some of the political defeats of the primaries ," she continued, "You can correct a political problem. It's awfully hard to correct a perception problem that is 180 degrees wrong."

Mondale has had other moments of personal trial in his roller-coaster quest for the presidency.

In the primaries, he was rudely stripped of his "inevitablity" -- then nearly stripped of his candidacy altogether -- by the surprsing challenge of Sen. Gary Hart (D-Colo.).

Back then, with primaries coming once a week, and with no time to get his feet back under him, Mondale talked of feeling like was "caught up in a tornado."

But he found a way to reach into his gut with classic, plainspoken what-you-see-is-what-you-get appeals to the core Democratic constituencies.

He gave some of the best speeches of the campaign. And he impressed aides with his "obsession," as Isaacs described it, that his children saw him as a man who was fighting back, who was not prepared to accept history's judgment of that particular moment.

The narrow escapes of the spring have steeled Mondale for the disappointments of the fall.

"When you suffer as many political obituaries as he did, you become quite optimistic about the potential for political resurrection," said speechwriter Martin Kaplan.

Through the worst weeks of the fall, Mondale has been steady; it is not in his nature to become hangdog or to complain.

His one "blowup" of the campaign came in mid-September, when poor advance work deposited him at a plant gate in Green Bay, Wis., with a bare trickle of workers walking past, and with the candidate left to pose for pictures with a sample of the celebrated product manufactured inside -- Charmin toilet paper.

No more hokey events, Mondale decreed at a tense staff meeting that night. It has been town meetings and noon rallies ever since.

The candidate and staff consoled themselves with the hope that once the electorate became "engaged," once they started paying attention to Mondale's arguments, the race would tighten.

Still, the polls have been hard to ignore -- and they have not been kind. Last weekend, at a black-tie fund-raiser in Minneapolis attended by many of his oldest friends in politics, Mondale briefly hinted at some of the wear of his longshot candidacy -- though he did so with his trademark dry humor.

"For a while there," he told his friends, "the polls were so low I was beginning to have doubts about Joan."

Then to the same audience, Mondale went on to describe, with a kind of awe-struck wonder, the crowds he'd drawn in the week after the debate.

"It was like somebody suddenly turned the switch on," he said. He marvelled at crowds "further than the eye could see" in Madison, Wis., and "the biggest rally they ever had" in downtown Pittsburgh.

He told of seeing his first "Republicans for Mondale" sign in Philadelphia and marching in the Columbus Day Parade in New York with "sidewalks lined six and eight deep."

"I even got more applause than Sophia Loren," he said of the New York event. (A day later, he admitted: "Somebody said that was the first time he ever heard me lie.")

Heady stuff for a politician whose career has been marked, at every important turn, by appointment, rather than election, to office; for a candidate who never had the yellers and screamers out on the stump; for a candidate who jokes about his Norwegian reserve.

But, oddly, Mondale chose to cut short this romp through big, enthusiastic crowds so he could spend two days in North Oaks, Minn., last weekend, and four more here in Washington this week preparing for the debate.

To some, Mondale's scheduling was new affirmation that he lacks the raw energy that typifies so many top-level politicians. This is the man, remember, who took himself out of the 1976 race for the presidency because he didn't want to spend his life in Holiday Inns.

To others, it was also evidence that he is the rare politician who really doesn't feed off the adoration of crowds. Some on Mondale's staff said he was simply leaping at the chance to dig into foreign policy analysis.

For all the boning up on content he's done, Mondale's prospects tonight will hinge at least as much on style and tone, and the candidate has been spending hours refining his lines and gestures.

In Louisville, his triumph was a matter of finding a way to attack Reagan while remaining respectful, gracious, "big."

Mondale would like to walk the same line tonight, but it will be more difficult.

His best hope for victory lies in convincing voters that Reagan raises the chances of nuclear war. That's hardly the sort of broadside that can be lobbed in a loose, lighthearted no-offense-intended way.