For one brief shining hour and 40 minutes in Louisville, Walter F. Mondale's wildest dream came true. With an astonishing assist from an exhausted, nonplused rival, he succeeded in making Ronald Reagan the issue, stirring doubts that the president was too old for the job.

Mondale, redeemed and rejuvenated as a candidate, renamed his campaign plane "Louisville Slugger" and played to huge, suddenly euphoric crowds. It was springtime on the trail.

But reality and Reagan came back in a rematch Sunday. It is Reagan's luck that he is always judged on his own terms. All that was asked of him was that he get through a sentence without faltering. The evidence is the country doesn't care what he says, only that he look spry when he says it.

He was, in the Music Hall, his old self in all respects. He looked rosy and rested; he blithely strewed errors, indiscretions and zany ideas and liberally passed the buck for foreign-policy mishaps and disasters.

But he was also dispensing quips and quotes and one-liners and crisp comebacks, and he laughed the age issue off the election screen: "I am not going to exploit for political purposes my opponent's youth and inexperience."

As a debater, Mondale acquitted himself perfectly well. But the genial, nimble-witted, sure-footed self he presented in Louisville was supplanted by the cautious politician of melancholy memory.

The tone he brought to the first debate on domestic questions -- respect for the office of his opponent and fondness for the man -- eluded him in the discussion of foreign policy.

But he obviously did not want to tangle with Reagan at close quarters. He had points to make, and he often made them without regard to context. He was seemingly anxious to avoid the routine Republican rap against Democrats that they are weak on defense and soft on communism.

The haunting fear was clearest in responses he gave on the nuclear freeze. He was at such pains to emphasize his resolute refusal to ban any systems that could not be verified that he neglected to mention any that could.

At one point, he turned to Reagan and said, almost pleadingly, "Mr. President, I accept your commitment to peace, but I want you to accept my commitment to a strong national defense."

Reagan ignored the plea, as he did Mondale's demand for an apology from Vice President Bush for saying the Democrats had declared that Marines "had died in shame" in Beirut.

Mostly, Mondale acted as if Reagan were not there, despite his proximity. His mantra for the evening was "tough," and he tucked it in wherever he could. Ironically, Reagan amply confirmed Mondale's charges of inattention to detail and inability to accept blame. The president laid off the embarrassing CIA manual on U.S.-sponsored assassination in Nicaragua, putting it at the feet of "a gentleman down there in Nicaragua who is on contract to the CIA," thereby carelessly confirming that the CIA is running the secret war. He also fingered a shadowy somebody in charge of the operation -- and had to remove him hastily several paragraphs later.

He blamed the Marine massacre in Beirut on an unnamed "ground commander" who had defied the Joint Chiefs of Staff and presumably the commander-in-chief to keep the Marines at the airport. Col. Timothy Geraghty came out as an exemplary officer in other accounts of the tragedy.

He gave Mondale his widest opening on "Star Wars." The president summarily explained a trillion-dollar nuclear defense system that he is willing to share with the Soviets.

He has no idea "where it would be" -- "I am not a scientist." All he knows is that if we demonstrated it to the Soviets they would at last negotiate. Once we propositioned the Soviets, plainly the one question to be asked about the demonstration would be, "Your place or mine?"

Mondale forbore and addressed himself soberly to noting the impracticality and the cosmic expense of the daffy notion.

Afterward, the Mondale staff professed to be delighted with the ammunition that Reagan had provided them for the last days of the campaign, while the Reaganites purred that their man was tall in the saddle again.

The sense of a last, lost chance was heavy in the air when Mondale traveled to Philadelphia. Before a crowd estimated at 15,000, he attacked the president with a force that had eluded him when he was talking to the roughly 80 million Americans who had gathered by their sets in anticipation of a knockout blow that never came.

For Mondale, it's back to "rapping at the window, crying at the lock."