As President Reagan prepares for the last big political act in his final campaign drama, his advisers are outwardly brimming with confidence that he will triumph in the arena where he has won so often in the past.

"I've never known a greater political competitor," says his longtime friend and campaign chairman, Sen. Paul Laxalt (R-Nev.). "He always does best when he's backed into a corner."

Laxalt and Reagan's closest aide, deputy chief of staff Michael K. Deaver, believe that the president will come roaring back in Kansas City tonight in his second debate with Walter F. Mondale.

"He seems bouncier and much more ready than he was before," said Stuart K. Spencer, the president's senior political adviser.

These comments reveal what Reagan's operatives believe is most important about their candidate -- his character, his competitiveness and his intuitive sense as a communicator rather than his grasp of factual detail.

Gone is the belief that, with statistics, Reagan can show himself as a president who excels in the mastery of subject matter.

"What have you done to Ronnie?" Nancy Reagan reportedly said to White House aides when a shaken president left the stage in Louisville after a shaky summation that shattered even loyal supporters.

This time, say Reagan loyalists, it will be different. This time, as Laxalt said after the first debate, "we will let Reagan be Reagan." This time the Reagan managers will try to prepare their performer psychologically by having him address a partisan rally this afternoon to "get his juices flowing," as one aide put it.

The idea of letting Reagan be himself, even if that means putting up with some factual errors, appeals to his associates. They remember all the times before when Reagan was underestimated -- in his first campaign for governor in 1966, in the North Carolina primary in 1976 after he had lost a string of early primaries to President Gerald R. Ford, in New Hampshire in 1980 after an upset loss in Iowa to George Bush.

"What happened in the first debate was almost providential," one Reagan intimate said. "He now goes into the second debate as an underdog and that's when he performs best."

Despite these expressions of confidence, there is mild anxiety within an inner circle that is confident of victory on Election Day but has begun to feel weary in the White House.

The jibes that Reagan, 73, is showing his age are resolutely denied but they have taken a toll on the staff, partly because of Nancy Reagan's vocal criticism of the first debate preparation. Others in the White House have expressed nostalgia or regret as they begin to realize that tonight's debate marks the homestretch of Reagan's final candidacy.

"It's the twilight of his political career, whatever happens, and there's a bit of sadness in that," a longtime associate said. "There isn't anyone quite like him. Once the votes are counted on Election Day, he'll never be campaigning for himself again."

Whether Reagan thinks this way or reflects at all about his final campaign is not known. He does not bare his soul, except perhaps to his wife. Like reporters and others who watch him from afar, the president's associates seek clues to Reagan's mood by observing his conduct.

What they have observed is a candidate passively accepting the recommendations of political advisers even while occasionally chafing at the restraints imposed by a cautious campaign of incumbency.

This has usually been Reagan's way. He was 55 and set in his ways when he first ran for political office in 1966 and was elected governor of California. Political details have always bored Reagan, and his early managers considered him a dream candidate because of his willingness to accept direction on strategy, scheduling and other aspects of the campaign.

Put simply, Reagan did what his managers told him to do. That was how he tried to run against Mondale after the Democratic convention in July.

"Coming out of the convention, Reagan thought and was advised that the way to play it was presidential, above the fray, and he did so," one campaign official said. "All of us, and him, warned ourselves about the dangers of being overconfident. But we lulled ourselves a little in underestimating Mondale . . . . "

Part of the strategy involved avoiding the media. Reagan last held a news conference on July 24. After news accounts stressed his isolation, the president reportedly favored another news conference. But his managers vetoed it as risky, and Reagan, as usual, accepted their decision.

Running as an incumbent, with a record to defend, also cramped Reagan's campaign style, because he relishes anti-government themes. Californians in his entourage are reminded of 1970, when Reagan ran for reelection as governor.

It was not one of Reagan's better campaigns. Though his opponent, Democrat Jess Unruh, had high negatives, started badly and was underfunded, he cut Reagan's winning margin in half. Unruh repeatedly urged Reagan to debate, without success.

This summer Reagan's caution was intensified by a belief, shared by White House chief of staff James A. Baker III, that he faced a close election even though his polls showed him far ahead. Ever since he allowed himself a victor's pose before the cameras in the 1976 New Hampshire primary, a contest he wound up narrowly losing to Ford, Reagan has been superstitious about taking victory bows prematurely.

"His superstition and Baker's caution were reinforcing," a campaign aide said. "They just wanted to sit there in the Rose Garden and let the election take place."

But the Oct. 7 debate with Mondale changed Reagan's style, in a manner that Laxalt and Spencer think has been highly beneficial. One Republican source said he is convinced that Laxalt never would have voiced his subsequent sharp-toned criticism of the White House staff "unless he had a good idea of what Reagan was feeling."

What Reagan likes to do best in any campaign is polish speeches and perform, and his staff finally turned him loose after Louisville. Reagan was rusty, but the political value of unleashing him was demonstrated on Oct. 12 in Ohio, where Reagan five times gave a new stump speech prepared by veteran campaign speechwriter Ken Khachigian. Reagan polished and repolished it as the day went on. By his last stop, he delivered his most effective address of the campaign.

Allowing the performer to perform has been the strategy pursued by his managers to prepare him for the second debate. Reagan is concerned now about getting his lines right, rather than remembering statistics.

While there is a potential peril in this, the new approach gives him the chance to be himself in tonight's debate -- and few excuses if he fails.

Said one top adviser Friday: "Mondale's really irrelevant now. Reagan is far ahead and will win unless he fouls it up. What will happen in Kansas City is a contest of Reagan versus Reagan."