Jimmy Carter found his voice again last night.

Whether he also found an energy program that will cure the crisis he sees as a symbol of the nation's troubled spirit is another question.

The speech he delivered to an expectant nation, after 10 days of isolation, may or may not rescue his faltering presidency, revive public confidence and set America on the road to energy independence.

But it will surely go down in history as one of the most extraordinary addresses a chief executive has ever given.

It was a speech that only Jimmy Carter could have given - the kind that made him so distinctive and effective a candidate in the early months of 1976.It was the kind of speech he had failed to give in his first 30 months as president.

It dealt with what he believes are the fundamentals. The character and spirit of the American people. The values that built the nation. The trust that must exist between the government and the governed.

The atmosphere he created from the desk in the Oval Office was that of the living rooms in Iowa and New Hampshire. The rhythm of his speech - the emphases and the pauses - made you think that in his mind's eye, Carter was not seeing the camera and teleprompter, but the faces of those early audiences - waiting for the nod of the heads that told him they had understood and agreed with his last point.

As a result, in this most critical speech of his presidency, he delivered his text more effectively than he has ever done before. He avoided the sing-song rhythm, the misplaced stresses, and the falsetto squeaks that have marred past performances.

His voice was strong throughout, and, on occasions, ringing.

What he said was the kind of thing Americans have never heard from a president before. "This is not a message of happiness or reassurance," he declared, "but it is the truth and it is a warning."

Some of the hardest truths he uttered were about his own performance as president-reflecting the frank criticism he had invited and received from the outsiders he consulted at Camp David the previous 10 days.

He quoted the blunt comment they had made to him that "you're not leading this nation - you're just managing the government."

He set the stage for the promised shakeup of his administration by quoting the chilling comment that "some of your Cabinet members don't seem loyal. There is not enough discipline among your disciples."

And he acknowledged that he has so far failed, not just to deal with the energy crisis, but with the distrust of Washington and the national government that, as a candidate, he said he would strive to overcome. "the gap between our citizens and our government," he confessed, "he never been so wide."

But in a passage that is critical to his prospects for political rehabilitation, he tried to put himself again on the side of the citizens affronted by the spectacle of Washington's paralyzed government.

"What you see too often in Washington . . . is a system of government that seems incapable of action . . . a Congress twisted and pulled in every direction by hundreds of well-financed and powerful special interests . . . Often you see paralysis, stagnation and drift. You don't like it, and neither do I."

These criticism were common refrains of the Carter campaign, but they came last night, not from an outsider, but from the head of government, a president who has failed thus far to establish a comfortable working relationship with a Congress of his own party.

Early in his term, he had threatened to go over the heads of Congress, but was usually dissuaded from doing so. Last night, when his own standing in the country had never been lower, he reached into the living-rooms with a fervent plea for help from the homefolks in moving Congress to act.

The energy program he outlined last night - and will detail in speeches and briefiings today - was, as his aides had indicated in advance, devoid of dramatic surprises.

The keystone pledge to hold oil imports to the 1977 level is a bite on the bullet - but not the kind that will break any teeth.That was the peak year of imports. Last year was lower and this year is expected to be lower again. The pledge to cut imports by 50 percent by 199o is tougher - but it is also more remote.

The program for production of synthetic and exotic fuels is ambitious - but its total costs were left unclear by Carter last night. Nothing was said of the future of nuclear energy-a highly controversial question. Nothing was said of the future of price controls on gasoline and crude oil.

What Carter did convey - more clearly and probably more convincingly than in all his previous discussions of the energy issue - was the fundamental fact the American people have been most reluctant to accept: "The energy crisis is real . . . It is a clear and present danger to our nation. These are facts," he said, "and we simply must face them."

It is his belief that by accepting the challenge of energy dependence as real and striving to overcome it, America can regain its lost sense of purpose and find a cause in which to unite.

There is, however, as he well knows, no issue which is more divisive - between the regions, between producers and consumers, between regulators and enterpeneurs, between free-market advocates and believers in federal controls.

But it is on this issue that Carter has staked his presidency.

A more cautious politician would probably have chosen another issue - almost any other issue - by which to be judged.

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If Carter had played politics by cautious, conventional rules, he would almost certainly have spent the last four years running a peanut business in Plains.

It was only by announcing for president earlier than almost anyone else, by daring to run in all the primaries, and definitely stating from the outset that - when he was at 2 percent in the polls - "I do not intend to lose," that Carter made it to the White House.

This gamble has got to be bigger than any of those he has taken before.

But he covered his bets with a double good-luck-charm. He brought the message down from Camp David, the site of the greatest success of his presidency, the negotiation of the Egyptian-Israeli peace agreement.

And he delivered his speech, as he reminded his listeners in the opening sentence, on the third anniversary of his acceptance of the Democratic presidential nomination.

On the same date in 1976, he had said that "our nation has seen a failure of leadership. We've beenhurt and we've been disillusioned . . .We've been without leadership too long."

Last night, he came as close as anyone in office can to confessing that the same failure had characterized his own conduct of the presidency.

But he asked people to believe that was about to change. "I promise you," he said, "that I will lead our fight. I will enforce fairness in our struggle. I will ensure honesty. And above all, I will act."

The polls and his personal counselors have been telling him - with increasing urgency - that action is what the public demands.

Last night, the people heard the promise.

Starting today, they will measure again the performance. CAPTION: Picture, President Carter in his TV address: "The gap between our citizens and our government has never been so wide." AP