In the 90-minute debate that could determine the outcome of his uphill battle for reelection, Jimmy Carter accomplished almost every objective except the most important one: the destruction of Ronald Reagan's credibility as a potential president.

For most of their nationally televised debate, Carter's challenger was kept on the defensive -- explaining that his views on arms-control, military weapons, Social Security and domestic programs were not the "very dangerous, disturbing" and even "radical" notions that the president said they were.

In a confrontation where most of the time was spent on Carter's preferred issues -- and not the economic record of the last four years, on which Reagan would have preferred to focus -- the incumbent repeatedly managed to work in a partisan appeal to his fellow Democrats and to aim special messages at such key constituencies as blacks and Hispanics, the South and the Jews.

But in the end -- when Reagan managed in his summation to ask voters the question Carter most ardently wished to submerge, "Are you and your family and your country better off than you were four years ago?" -- the challenger was not only still on his feet but in contention for the White House.

The minimal objective defined by such Reagan strategists as pollster Richard Wirthlin was that the GOP nominee look like "a reasonable man" and not "a dangerous personality." By that standard, the former California governor earned much more than a passing grade.

Reagan used all the skills acquired in 40 years before the cameras -- shrugs and smiles and easily inflected small jokes -- to tell the viewers that the portrait of him Carter was drawing, that of a weapons-prone right-winger, equally heedless of the threat of nuclear war and the aspirations of women and minorities, was a political caricature.

Reagan's advisers went into the debate confident that their candidate had an edge on Carter in electoral votes and a chance to wrap up the election if he avoided obliteration tonight. The Reagan camp passed off as insignificant Carter's consistent show of greater expertise on government programs, economic statistics and energy production figures.

Less easily dismissed was Carter's skill in developing what has, until tonight, been a fitfully communicated message that -- for all the surface blandness -- Reagan's underlying attitudes are a source of danger to the nation and damage to most working Americans.

Carter's most direct hit came in the middle of the debate, during the emotionally charged discussion on the strategic arms limitation treaty (Salt II) and the whole question of a nuclear arms race.

After Reagan had attempted to rebut Carter's claim that such an arms race would be "a very dangerous and disturbing thing," Carter came back with an answer that used the word "dangerous" three more times.

Finally, he said, "it is extremely dangerous to hear this belligerent tone, though said in a quiet voice."

For close to half an hour in the heart of prime TV time, Reagan found himself constantly on the defensive, as a series of questions on minorities, energy and Social Security bracketed the central exchange on nuclear weapons, war and peace.

It was that fact that pleased the Carter strategists. The president "didn't do everything I wanted him to do," said pollster Patrick Caddell, "but he did 75 percent of it."

Caddell said that Carter's message was aimed at specific constituencies in the debate audience -- particularly the "college-educated, Democratic-inclined women, who are unhappy about the economic squeeze of the past four years but nervous about Reagan's views on war and women's rights."

The debate was helpful to Carter, Caddell said, because so much time was spent on all the issues except the economy, which is Carter's biggest liability.

"When Reagan spent two full answers trying to get right on the Equal Rights Amendment, I couldn't believe it," Caddell said.

It was Carter's ability to cut off discussion on dangerous topics and to shift the ground to areas where Reagan was vulnerable that accounted for his political edge in the debate.

Both men started off a bit shakily, which was not surprisingly considering how much they had riding on the debate. They sparred on the opening question on international policy, with Carter getting control of his voice and facts a bit quicker than Reagan, but neither gaining a clear advantage.

In the camera work that continued all evening, both men appeared composed in the close-ups, but Reagan, of course, was the dominant figure with his greater height and bulk, in the longer-range shots.

Reagan had the better of the exchange on the second question -- asking for anti-inflation strategies. While Carter was clearly on the defensive, he managed to introduce the theme he developed continually -- hat Reagan's economics were just a dressed-up version of the "heartless" Republican policies of the past.

To the disappointment of some of his backers, Reagan failed to grasp the opportunity offered by the next question, on the needs of the cities and the poor to demonstrate clearly that Democrats have no monopoly on compassion. t

Instead, he talked in hard-to-decipher terms about a specific legislative proposal to attract industry to center-cities and gave Carter an opening to suggest that only recently has Reagan even developed a "knowledge of racial problems" in America.

Then came the period when Reagan was on the defensive, constantly objecting that Carter was distorting his views: "I'd like to correct that misstatement . . . that is a misstatement of my views . . . There you go again."

Earlier in the campaign, Carter had been forced to apoligize for some of his harsh anti-Reagan rhetoric. Tonight, obviously cautioned on the risks of such conduct, he stayed on the safe side by offering negative characterizations of Reagan's "attitudes" and "underlying views," rather than of Reagan the man.

This tactic may have been dictated by the obviously danger of an anti-Carter backlash if he further personalized the assault.But from the perspective of the Reagan camp, it almost guaranteed that their most precious commodity -- Reagan's personal credibility as a potential president -- would survive tonight's encounter intact.

And that is what the Reagan people were saying when it was all over, and what Reagan himself obviously felt.

The challenger was at his best in his final comments of the night. "Ask yourselves in the polls," he advised prospective voters, "are you better off than you were four years ago? Is it easier to buy what you want? Is there more or less unemployment? Is America more or less respected in the world?"

And then he went back to what all the polls show is the prime support for his plausibility as a future president -- his record as governor of the minination called California.

"My economic program will work," he said, "because we did it in California."

Carter did not destroy that proposition any more than he destroyed Reagan's personal credibility in tonight's debate. In an election so close to the grasp of the challenger, that failure may count for more than all the good the president accomplished tonight.