Jimmy Carter is a man given to rebirths. He is a man who has the capacity to stop in midcrisis, take stock and set out on a new course with a different purpose.

He did that with his personal life in 1966 when he suffered a traumatic defeat in his first bid to become governor of Georgia, and he is trying to do that with his presidency this week. In a dramatic way, unprecedented in style and scope, and with a reelection campaign just months away, Carter is promising Americans a presidency headed by a man eager to lead instead of one anxious to find out how it works. He is talking of energy and the economy, but he also is talking of himself, because he has concluded all are entwined in what he has called a "rebirth of the American spirit." His new approach means, in part, the end of the clockwork presidency.

The rebirth of the Carter presidency has its roots in a memorandum given Carter last April by his public opinion adviser, Patrick Caddell. It was a portrait of an America drakened by gloom and by dissillusionment of unprecedented proportions in modern times.

The memorandum left Carter some-what stunned and shaken, according to advisers close to him. It marked the beginning of an introspection that took him to the mountaintop of Camp David and then, last night and today, to the nation.

Caddell's public opinion data showed that the percentage of Americans who viewed their personal futures pessimistically had doubled - to 32 percent - in the last six months. The increase was more than the total increase of the past 10 years, and far exceeded the previous high of 24 percent, recorded in 1974 when Watergate was driving Nixon to his depths, inflation has soaring to its first double-digit range and the shock of the oil embargo still was being felt.

There was more.

Almost half the country was now answering that they were pessimistic about the long-term future of the United States. Only 6 percent felt the nation could make progress in dealing with such crucial problems as inflation and high taxes. In 1960 more than 60 percent felt the country could progress in these areas.

And when asked to rank the past, present and future on a scale of one stark progression of pessimism. They ranked the past at 5.7, the present at 4.7 and the future at 4.6.

For the first time in history, Caddell wrote, "We found a mean line that was a straight negative curve."

These were the hard facts Carter had before him when he talked with his visitors to Camp David. Last night in a speech of extraordinary candor, with unabashed self-criticism, Carter reported to the American people what his invited counselors had told him.

At one point the president quoted what a southern governor had told him: "Mr. President, you are not leading this nation. You're just managing the government."

And in the recesses of the West Wing of the White House, just a few feet from the closed door of the Oval Office where the president was giving his address, one of the president's top advisers said, "This is the Jimmy Carter that many of us haven't seen for a long time. He sounds confident and aggressive. He understands the deep feelings of this country better now - and he understands where he went off track."

Part of the problem, he knew, was what he had called the malaise that had gripped the country. But part of the problem was something else - it was part of the reason why 80 million people had tuned in for his first energy speech, but just 30 million had tuned in for his fourth. Part of the problem was Carter.

He had disdained repeated staff suggestions, such as one from communications adviser Gerald Rafshoon, that he use a speech coach to improve his oratory style. He had stuck in the first years of his presidency to that southern churchy style, where he would let his voice tail off at the end of every sentence, a habit that connoted reverence but not confidence. Carter even disdained suggestions that he at least practice what he preached, fixing a scowl on anyone who dared suggest a rehearsal. But that was in the first Carter presidency, not the one that was inaugurated last night.

Carter rehearsed, videotaped, analyzed and rehearsed again before the speech. What Americans saw last night and again today in his appearances in Kansas City and Detroit was a Jimmy Carter striving valiantly to sound as bold and as confident as his words. He had worked at ending his sentences with a firm voice and clipped accent. He had mastered such gestures as the clenched right to the desk top, and he virtually had conquered that habit of grinning at things that are not funny.

So it was the Carter was able to put new vigor into what was, for him, an old message.

"We need to have confidence in ourselves. Our nation is strong - economically the strongest nation on earth. Richard Nixon didn't hurt our system of government. Watergate didn't hurt our system of government. The Vietnamese-Combodian war didn't hurt our system of government. There is still a basis on which we can predicate answers to complicated questions and bind ourselves together and face the future with confidence."

That was Carter speaking back in his campaign in a speech in Alabama. It was the same let-us-conquer-malaise message that he has been giving this week. The thing that Carter and his top advisers have come to see is that the president, not just the nation, had lost the way.

Carter found it on the mountaintop. He found it not just from listening to invited guests tell him what was wrong, but from long walks in the woods and from reading such writers as John Gardner on morale and James MacGregor Burns on leadership.

"The president began to understand that the crisis at hand is not limited to a political crisis, but [is] a leadership crisis," said one of that inner circle of advisers with him throughout his Camp David conference.

"I think we have seen both the rebirth of the American spirit that he talks about and the rebirth of the Carter presidency as well."

In style, scope and effort expended, that is certainly true. Whether Carter's reborn presidency proves any more effective or electable than his first-born presidency will only be known months fron now.

Carter has given the country energy policy proposals that are no surprise and promise no quick fixes. But he has given the country one more pledge of leadership and that can, in the long haul, be his strongest asset.

His speech last night was not remarkable because of any truly great lines. His speech was remarkable simply because he gave it and he gave it well.

EPILOGUE: It is almost midnight Sunday and, borrowing a phone in the office of press secretary Jody Powell, Patrick Caddell dials the executive mansion and gets the president on the line.

Caddell is reporting good news. The research firm of R. D. Percy and Co., owned by the son of Illinois Republican Sen. Charles Percy, has polled Seattle viewers on the Carter speech and found the response overwhelmingly favorable - the highest response for any TV half hour they have ever measured. Caddell says Carter scored positive ratings in the high 70s and high 80s on 10 questions.

Carter listens in silence and replies softly: "That's great." His voice, Caddell notes, is happy but very drained. Not the commanding voice from the television tube, but that of a man who has just run a very long race - and, Caddell adds, has won.