In the political jungle that Washington can be in the fall of an election year, there is one absolutely unscientific and totally reliable test for determining which candidate really did win a presidential debate: in the 72 hours immediately following the debate, that candidate is the winner whose previously unnamed advisers candidly, if bashfully, admit in the press that, yes, they were intimately involved in preparing their guy for the debate. Advisers for losing candidate-debaters are invariably more discreet. Just as in the fall of 1980 we learned that young David Stockman had brilliantly played the parts of both John Anderson and Jimmy Carter in Ronald Reagan's warmups, you can bet that this week we will be introduced to the Mondale briefers.

Sometime before 11 o'clock EDT on Sunday night, Oct. 7, Walter Mondale underwent an important public transformation. Before our very eyes, Mondale the Loser turned into Mondale the Underdog. This is potentially important, because Americans are funny people. While we alternately snicker or chuckle at losers, we root for likable underdogs. After Sunday night, Mondale is no longer a wimp. He went toe-to-toe with our president -- who is, it is acknowledged, tough as nails -- and won a near-unanimous decision. In the colorful boast of his campaign pollster, Peter Hart, "Mondale proved his mettle, while Reagan lost his Teflon."

Reagan spent most of the evening on the defensive, insisting that he was not the sort of fellow -- as Speaker Thomas P. O'Neill implied during the 1982 campaign, when unemployment was at a 40-year high -- who delighted in throwing his disadvantaged contemporaries off Social Security. In the words of one Republican manager, the debate was "an unmitigated disaster for Rea gan. The president did not look to be in command. He lost control of the agenda."

Reagan himself, with his selection of words before his closing statement, seemed to raise the age issue, which his halting performance is sure to resurrect in the campaign's closing weeks. Reagan said to moderator Barbara Walters: "I'm all confused now," an adjective often used euphemistically to describe the forgetfulness of older people.

By Monday morning, Democratic candidates and managers were smiling for the first time since the San Francisco convention. As one southern Democrat put it: "The mood is different. Mondale rehabilitated himself and Reagan is no longer seen as a superman who will carry on his coattails three dozen Republican House candidates. From a Republican manager: "There goes Chuck Percy," referring to the Illinois GOP senator, whose supporters had been counting on a strong Reagan effort to help the Foreign Relations Committee chairman against popular Democratic Rep. Paul Simon.

While Mondale's performance was better than Reagan's, it was far from unflawed. Still missing is any inspiring vision or challenge to Americans. But Mondale managed to be aggressive without being abrasive, while simultaneously conceding the president's immense personal appeal to voters ("I like President Reagan" should have made Mondale's nose grow three inches) and seeking to persuade Democratic voters to come home to the Old Party (pointing out that he, not the president, had worked for John Kennedy's election in 1960).

Once again the presidential debate has changed the conventional wisdom, if not the dynamics, of a presidential election campaign. The Reagan managers will now have to confront a suspicious electorate about their previous strategy of keeping the president under wraps. Democrats will now be openly asking whether the presidential helicopter engines had been turned on so that the president could not be heard by voters. Mondale is now the Underdog, with four weeks to go.