WATCHING SUNDAY'S debate between President Reagan and Walter F. Mondale was the political equivalent to watching Charles Bronson execute vigilante justice on the guy who raped and killed his wife and daughters.

There is much the same vicarious and slightly evil satisfaction of watching Walter F. Mondale turn the tables on the Great Communicator that some down in Florida feel when the latest serial killer gets fried in the old electric chair. 'Fry' is the operative word here, because Mondale did to Reagan in 90 minutes what Democrates, the press, and just about everyone who believes an elected public servant should be held accountable to his constituents, have failed to do: make him answer for himself.

What sweet revenge to see Reagan--whose affirmations of Ma, apple pie, and all values American, had taken the place of intelligent argument--beaten at his own game by a man who was ready to crucify himself on the cross of rational discourse. What guilty pleasure to watch the President of the most powerful country in the free world fumble with his adjectives, sweet through pregnant pauses, and spurt out meaningless figures when faced with the simple question of what he felt about abortion.

This is not to say that Mondale performed with a rhetorical virtuousity anywhere akin to the masters of yesteryear. He still plods along in a monotone, his laughter is still awkward, and he continues to suffer from a self-seriousness worn only by the truly indignant ("It's not fair...").

But this is small price to pay to restore to the White House a President who, at the very least, does not pander to our own worst instincts. It is small price to pay to rid the country of a man whose smug self-assurance thinly veils his daily denial of the complex, amoral, often unpleasant nature of everyday life.

Before last night, President Reagan never had to concede that, yes, not everyone is prospering in a year where Americans won a ton of gold medals at the Olympics, And that, no, abortion was not quite as simple an issue as he wished it would be.

Before Sunday night, President Reagan never had to defend his record for more than the minute it takes him to walk across the White House lawn to his helicopter. Unlike the past, President Reagan could not pass off the big holes in his political safety net with accusations of press persecution, or silly anecdotes about the woman in Lubbock, Texas, who used her $10 food stamps to buy grain alcohol.

Instead, he had to look Presidential and he was trapped by his own lack of intellectual vigor as well as a singular lack of experience in defending his government rather than attacking someone else's. How much longer could be retain credibility by dumping every failure on the Carter Administration when he has run the show for nearly four years? How Presidential could he look by rationalizing his every gaffe by blaming Congress? Not very, and Reagan was caught in his own Catch-22.

Things should get even worse for the President on October 21, when the subject turns to foreign policy. He will have a tough time keeping his smile when he has to rely on the Grenada invasion for 90 minutes as his sole foreign policy success. He could do little to defend his economic program-considered his strongest point. He will look even older and frailer when he has to answere for the first time in public, for three Lebanon bombings, his absurd nuclear posture toward the Soviet Union, his inability to assert American influence over anything larger than a golf course, and what amounts to terrorist activity in Nicaragua.

If any sort of shot-in-the-arm is needed for the much-maligned Mondale candidacy, these debates should provide it. Though the shrinking ranks of college-age voters who remain Democrats went largely for Sen. Gary W. Hart (D-Colo.) in the spring primaries, the time has come not only to support Fritz Mondale, but to support him with open arms. There is no excuse to hide behind the traditional criticisms of Mondale--he's boring, backward-looking, etc.--because, as Sunday's debate showed, the perception of glamor and excitement is about as deep as the celluloid it's projected on and does not stand up to a strong dose of reality.

When Mondale threw Reagan's "There you go again" line back in his face, it was nothing less than a victory over the network of euphemisms, symbols, and downright lies that has served as the Reagan Presidency. Sweet revenge indeed.