Can there be a meaningful public life for Gary Hart after Donna Rice and his graceless withdrawal as a presidential candidate last spring? Indeed there can, if Hart's first outing as a prematurely elder statesman can be taken as a valid test.

Proceeding from his sometimes embarrassing and singularly unpersuasive encounter with Ted Koppel on ''Nightline'' a week or so ago to a more serious, highbrow setting at a dinner meeting with the World Affairs Council of Philadelphia the following night, Hart unburdened himself of a heavy half-hour discourse on the future of U.S.-Soviet relations.

Who cares? I wondered. Could the passage from ''Monkey Business'' to so prestigious a platform be that simple? Apparently, you had to be there, according to the Philadelphia council's president, Mrs. Buntzie Churchill. She filled me in on the background and on the full flavor of the event -- the turnout, the audience reaction, the content of the questioning, the behavior of the media and other details missing from mostly meager news accounts.

In the process, she blew a large hole in my theory of the case, which was that Hart's judgment on anything would not count for very much after his confessions to Koppel of his ''bad judgment,'' ''serious mistakes'' and ''sins'' -- his acknowledgment, finally, that, yes, he had ''not been absolutely and totally faithful'' to his wife. Still less did I think that he could successfully play the powerful role he sees for himself as a noncandidate.

Thoughtful Americans, I figured, could hardly be impressed by the argument that his transgressions were, well, relative. They were ''not as bad as some others'' -- the lawbreaking, the lying to Congress, the shredding of documents and the dispatching of U.S. troops ''into combat to die unnecessarily'' that he laid on the Reagan administration.

Just as surely, the American public would find the flaw in his proposition that ''public interest ends where a person's personal life and private life does not affect his or her performance in office.'' That has always struck me as precisely the point about Hart's peccadilloes: that they not only did cripple his performance as a candidate but could ''affect his performance in office.''

From the moment he put his political fortunes so recklessly at the mercy of ''this attractive lady whom he had only recently been introduced to,'' as he put it, he was opening himself up to blackmail and scandal, from who knows what source or at what point down the road. Potentially, he was putting even a Hart presidency at risk.

He had, then, flunked his own ''public interest'' test, which probably explains why -- before the Koppel show -- the Philadelphia council had applications for press tickets from more than 150 national news organizations. What it doesn't explain is what happened after Hart's ''Nightline'' knockdown of rumors that he just might re-enter the race had taken much of the fun out of the Gary Hart story.

What happened was that the no-shows in Philadelphia were the national news people; the press section was almost empty. But the hall was jam-packed -- the biggest turnout ever, says Churchill, except for in-cumbent presidents. Sensation-seeking could explain it, except that scandal, judging from the questions, was not what the audience was curious about.

''Every question was on substance,'' says Churchill, meaning foreign-policy issues such as Nicaragua, the world economy, ''Star Wars.'' The rough stuff came on Hart's arrival and departure, when a shouting, shoving posse of would-be Sam Donaldsons bombarded an unresponding Hart with Topic A (as in adultery).

Now, it's true that the first stop on Gary Hart's roadshow to rehabilitation is hardly conclusive. This wasnot Middle America speaking. It was an issue-oriented, some would say elitist, audience, given to a certain civility.

But when you have said all that, there must be some room left for deeper meanings. Could it be that this Philadelphia story tells us something intriguing, if tentative, about the American body politic -- something about the appetites of those who consume as well as those who report the news, something about a hungering for substance as well as for sensation? It may even say something about the future for Gary Hart as he proceeds through a schedule of two dozen more speeches -- if he has been careful to pick his spots.