In a unique debate that turned into a test of Sen. Dan Quayle's presidential qualifications far more than his toe-to-toe ability against Sen. Lloyd Bentsen, Quayle easily met the original modest standard set for him at Omaha -- except for a mistake that exposed him to a brutal Bentsen rabbit punch.

Dogged by controversy over youth, experience, Vietnam War record and academic standing, Quayle, 41, drew the focus of the press panel's attention to a single, highly subjective question asked and asked again in speculative fashion: Was he qualified to succeed George Bush in an emergency?

It was on the third probe that Quayle, with a lack of precision typical of his vice presidential campaign, exposed himself to Bentsen's rabbit punch. He told his questioner that he had as much experience ''in the Congress'' as John F. Kennedy had when he ran for president in 1960.

Up to that point, Quayle had avoided anything close to the naive blunder that Democrats were praying, and Republicans fearing, would escalate the qualification factor into an issue that might damage Bush's quest for the presidency. Bentsen brilliantly seized the moment with his cutting rebuttal -- ''Senator, you're no Jack Kennedy.'' That sought to convert Quayle into a braggart brazenly comparing himself to the revered JFK. Quayle was momentarily stunned, but his comeback -- tight-lipped and angry -- was entirely to the point: ''That was really uncalled for, Senator.'' In the short time remaining, he visibly sagged.

Not for several days will it be known whether Bentsen's attack, repeated endlessly in television sound bites, might be a ''defining event,'' pollster Peter Hart's phrase to describe a sudden political change that affects an election result. On the other hand, if voters decide that the debate as a whole showed that Dan Quayle has a clear understanding of major issues, Bush's solid national lead in the polls will not be affected and Michael Dukakis will confront deepening despair.

It became immediately clear Wednesday night that the debate would demolish conventional wisdom that all Quayle needed for survival was to stop the nonsense talk that marred his campaign the past six weeks. He was held to a higher standard than any contestant in the short history of such debates, far higher than the less experienced Geraldine Ferraro in 1984.

The pressure came not from Bentsen so much as from the media panel. In a relentless effort to test Quayle's fitness, the panel peppered him with the toughest questions ever thrown in any such debate. Ferraro, who had only six years in Congress, was not asked a single question in her debate with Bush about her ability to succeed to the presidency.

Quayle's debate handlers may have overlooked the what-to-do question about succession, although his initial response seemed adequate.

Bentsen, widely viewed as a highly experienced and skilled political figure, was not even asked the question. Not explicable, however, was the failure to probe his vulnerability as a running mate in heavy disagreement with his chief on many issues.

But the fact is that the prime target in the ordeal of Dan Quayle was not Bentsen but Quayle. From that uneven playing field emerged Quayle's only real misstep as he grappled for the third time with the qualification question.

Quayle's problem far precedes the debate. It started with Bush's own surprise decision to shove the young Indiana senator headlong into the media pressure cooker in New Orleans without forewarning. The month following his selection planted seeds of doubt, some large, such as his nonsensical answer to a question about the Holocaust, some small, as in his confusion between Plato's Republic and Machiavelli's Prince in a recent interview with Michael Kramer of U.S. News and World Report.

Last week, after a peaceful fortnight, Quayle emerged from his handlers' cocoon and seemed primed for more exposure to the media. If now, following the debate, he becomes the first running mate in history actually to threaten the election of the man who chose him, the blame will be George Bush's as much as Danny Quayle's.