Not even President Reagan is claiming victory in his Sunday night debate with Walter F. Mondale.

The morning after his unexpectedly unnerving encounter with a man who is supposed to have one foot in the grave, the Gipper told a huge rally here, "I don't know whether I won or lost."

Another equally rueful assessment verdict had come from White House image-master, Michael K. Deaver, as the debate audience was still filing out of the Kentucky Center for the Arts in Louisville Sunday night.

"They both did well," he said.

In light of Reagan's hitherto unbroken string of debate wins, it was a virtual admission of defeat.

What was supposed to have been a meeting between a soaring eagle and a wet blanket turned out altogether differently.

In the sunny Carolina morning, among fierce, flag-waving partisans, the judgment was the same: sombre talk of the champ being "tense," "at a loss" and "groping for words."

One middle-aged man said sadly, "Something must have gone wrong backstage."

Nothing went drastically wrong onstage. But wan White House post-mortem boasts -- that "Mondale never landed a knockout punch" and that the president "held his own" -- told the story. Reagan was never really in charge.

The old actor, a ghost of his 1980 self, missed cues, flubbed lines, lost his place. In the antiseptic blue-and-white confines of the center, without the reassurance of balloons, bands, banners and a comforting script, he seemed lonely and afraid, just another politician clinging to his job.

The White House war-gamers had anticipated from their close study of videotapes of the debates during the fight for the Democratic nomination that the challenger would be "aggressive and mean."

Aggressive he certainly was, making his points on deficits and Social Security, pressing them home -- not always with regard to the questions. But he was not mean for a moment. Mondale was good-humored, gracious, humorous -- as Reagan usually is.

He took pains to express his respect for the president and his office. He said that he, too, likes Reagan. He was in control for most of the 100 minutes. He got the first laugh and made first use of Reagan's favorite themes of leadership and the Olympics. He cast friendly, inquiring looks at his opponent, gave him credit for this and that.

Reagan, pursing his lips, frowning, shifting uncomfortably, kept looking down at his lectern as if searching for some way to cope with the persistent, amiable intruder.

Finally, about midway, Reagan acknowledged the existence of an opponent he has ignored for most of the year. During an uncomfortable discussion about abortion, Reagan looked over at Mondale, a peer at last, and said in the direct address the stiff format was designed to discourage, "Now, I'm not capable, and I don't think you are . . . . "

Reagan had a flash of the jauntiness that marked his 1980 triumph over President Carter and quoted his own immortal line, "There you go again."

Mondale was waiting for him. He went on gleefully to put it in damaging context. Reagan, he said, had spoken the line when Carter accused him of wanting to cut Medicare, "and you went out and tried to cut $20 billion from Medicare."

It was game, set and match for Mondale.

The closing statements provided no contest. Reagan reeled off a string of numbers like an over-programmed computer. Mondale went inspirational with talk of arms control and "Star Wars" -- subjects not included on the agenda -- and an appeal to the altruism of an electorate infatuated with Reagan.

"Mondale was sharp," mourned a young mother of twins at the North Carolina Reagan rally. "If he looks that good in the next debate, it could make a difference."

The White House vigorously disputes the point. "So?" said Reagan campaign political director Edward J. Rollins when a reporter told him that Mondale had won. He added hastily, "Not that he did, but it makes no difference."

Richard G. Darman, the young lion of the White House staff, warned reporters to keep in mind "the strategic perspective."

"If Mondale won, which I do not concede," he said, "the gap is too great and the time is too short to be relevant."

But they all know that the debate -- if not Mondale -- raised the dread issue of Reagan's age. The 100 minutes drove home the fact that Reagan is 73 and not the man he was when he wiped up the floor with Carter in Cleveland four years ago.