When Ronald Reagan starts out today on the last campaign swing of his presidency, he will be lifting the final curtain on a remarkable political drama that has been playing to approving audiences and confounding the critics for nearly 20 years.

As usual, Reagan and his cast have high hopes that they have another hit on their hands.

But their jubilance is dampened by the realization that Reagan, after Tuesday, will never again take the campaign stage in his own behalf.

For the president, and those who have been with him through the years, the 15-stop, 10-state swing that will carry Reagan back home to California is the twilight campaign, the last hurrah. The aides are beginning to think about it, even though their candidate remains traditionally superstitious about premature claims of victory.

"There's a bit of nostalgia to all this," said deputy chief of staff Michael K. Deaver, a campaign aide when Reagan first ran for governor of California in 1966. "There's a relief, but there's a kind of sadness to knowing we're beginning the final chapter. I think election night is going to be very hard."

For the oldtime Reagan aides, who hold Reagan in affection and respect, it is difficult to think of a campaign without him. He was elected governor in 1966 and reelected four years later. In 1976 he fought President Gerald R. Ford to the wire before losing the nomination. In 1980, when Republican opponents were suggesting that he was too old to be president, Reagan routed the GOP field in the primaries and went on to defeat President Jimmy Carter.

There has been a recurring pattern to these campaigns. In all of them Reagan's competence was questioned. In all of them he stressed patriotic, evocative themes that celebrated the virtues of the United States and its people and questioned the wisdom of its government. In all of them he was underestimated by his opponents. And in all of them, Reagan's Election Day showing ran ahead of his standing in the polls.

Not even those close to Reagan knew why he was different, but all knew that he was. Over time, Reagan forged an almost personal relationship of trust with great parts of the electorate that defied conventional political wisdom and made him invulnerable to mistakes that might have sunk another candidate. Deaver believes that this relationship was reinforced by Reagan's wit and grace under pressure when he was wounded by a would-be assassin in 1981.

Twenty years ago, when he burst upon the national political scene with an electrifying televised speech for Republican presidential candidate Barry Goldwater, Reagan was known to Americans as an actor and television host. After Goldwater lost in a landslide, political advisers to two-term California Gov. Edmund G. (Pat) Brown thought they would be fortunate if their third-term opponent was Reagan, whom they scoffed at as a B-picture actor over his head.

In a historic political miscalculation, Brown's managers leaked damaging material about Reagan's principal GOP primary opponent to a columnist in hope of aiding Reagan, who won and then defeated Brown by a million votes. That night, at Reagan's victory party at the Biltmore Hotel in Los Angeles, a radio reporter who had known the winner in his Hollywood days said incredulously, "Can you believe it? This actor is going to be governor of California!"

Even his aides had difficulty believing it at first. Reagan struggled in the novice days of the governorship, where his performances included a memorable news conference at which he was asked to identify a single item of his legislative program and couldn't do it. But he persevered, and his second term was one of legislative accomplishment.

Lyn Nofziger, his first and most effective press secretary, believes that Reagan transformed himself from an uncertain governor operating on instinct to the confident president he is today without really changing his basic antipathy to government.

"The other thing that is most impressive about him is that as a human being he hasn't changed at all," Nofziger said. "When I first met him, he was a nice, decent man with no ego problems, and he wound up that way. How many people could you say that of who had been a movie actor, governor of the most populous state and president? He still kids around and jokes with you."

Nofziger's testimony to Reagan's essential unchangeability is echoed by Deaver and longtime political aide Stuart K. Spencer.

"He never became one of us," said Spencer, whose political acumen helped elect Reagan and then, on Ford's behalf, deny him the Republican presidential nomination in 1976. "He's never become a pol. When Bill Roberts and I were running Reagan around the state to see whether he would fly in 1966, we assumed the decision to run had been made in Reagan's head. I'm not so sure that was right. He was testing to see whether he was acceptable."

Today, as Reagan seems headed for reelection, he is behaving as he did in 1966, warning repeatedly against overconfidence and saying he is running as if "I'm one vote behind." He is also trying to help shaky Republican candidates.

In 1966, when polls showed his fellow Republican statewide candidates locked in tight battles, Reagan started talking up the GOP ticket at every stop instead of featuring his own candidacy. The other candidates also won.

In the next five days Reagan will travel to Massachusetts to boost the longshot candidacy of Republican nominee Raymond Shamie, visit Illinois twice in an attempt to rescue endangered Sen. Charles H. Percy and make a final foray into Iowa where GOP strategists say they worry that Sen. Roger W. Jepsen is going down the drain.

In these states, and in New York, Michigan, Ohio, Arkansas, Wisconsin, Missouri and California, Reagan will appeal for Republican victories. Unlike President Richard M. Nixon, who told author Theodore White after the 1972 election that it was "stupid" to assist Republican congressmen, Reagan is willing to invest his popularity in others.

As always, campaigner Reagan will, by choice, be untroubled by detailed assessments. His managers consider him an ideal candidate who goes where he is told to go and does what is asked of him.

When Spencer first met him in 1965, Reagan was writing his own speeches and answering his own mail, a practice that produced such documents as the now-famous letter quoted by Walter F. Mondale in which Reagan compared John F. Kennedy's ideas to those of Karl Marx. Even in 1966, Reagan wrote his own one-liners, including such notable ones as the comment on anti-war protestors: "Their signs said, 'make love, not war,' and it didn't look as if they could do either."

Today, although Reagan edits his speeches rather than writes them, he is most comfortable with basic restatements of American optimism, declarations that to his critics border on jingoism. "The United States of America was never meant to be a second-best nation," he says. "Like our Olympic athletes, this nation should set its sight on the stars and go for the gold."

He remains a master of the good-natured attack that focuses shrewdly on his opponent's vulnerabilities.

"If my opponent's campaign were a television show, it would be 'Let's Make a Deal,' " Reagan says in his basic campaign speech. "If the campaign were a Broadway show, it would be 'Promises, Promises.' And if his administration had been a novel, a book, you would have had to read it from the back to the front to get a happy ending."

For Reagan, the speech usually has been an end in itself, and so it will be in the final week of his last campaign. He will make 15 of them, winding up Monday at rallies in Sacramento, Los Angeles and San Diego. For Reagan and his entourage these last rallies are likely to prove bittersweet.

For the aides who have been with Reagan from the beginning, the reminiscences alternate with relief that it will soon be over. For the Deavers and the Nofzigers, the satisfaction at the distance they have traveled with Reagan will mingle with regret that there will be no other Reagan campaigns.

Nofziger was talking about it this week with Neil (Moon) Reagan, the president's older brother. Neil Reagan asked whether he should make the trip to San Diego, where his brother will conclude the campaign at the site where he made his final speech in 1980.

"You've got to be there, Moon," Nofziger said. "It's the windup, the last hurrah."

"You're right," Neil Reagan replied. "I think I better go."