Among the president's high command, the realization has slowly set in: they have taken what was a minor irritant and turned it into their own national nightmare.

The Billy Carter affair has now been mishandled to the point that some of those closest to the president are having doubts -- for the first time -- about whether this will be that final burden that costs their boss the presidency.

"The way the facts are coming out is the worst possible way," said one of the president's advisers. "And it's coming out at the worst possible time."

Another Carter aide said of the campaign's top command: "For the first time, I see real fear."

The president's White House and campaign advisers are not concerned that there is any evicence of crimes that would drive Jimmy Carter from the White House. They worry, instead, about the political repereussions that have sprung from this scandal -- and their handling of it.

In meetings early this week, the Carter advisers, mindful of the lessons of Watergate, emphasized to each other the importance of making all of the facts public immediately, leaving no revelations to be discovered and no explanation to chance. They then pull on a week-long performance that looked, for all the world, like the modified limited-hangout route, one more time.

On Tuesday, the Carter White House put out its official white paper that was purported to tell all, once and for all. There followed, one a day, a series of major revelations of crucial facts that were not contained in the initial White House explanation.

"It's not a problem of our intention, but of our execution," one Carter adviser said sadly, "just as it's been for the past three years."

Could the Billy Carter affair cause the president to lose the presidency -- and perhaps even the nomination that seemed numerically secure just a few days ago?

A senior adviser, who has been aggressively confident in the past, paused before answering. "We'll see," he said quietly, shaking his head. "Only time will tell."

In several crucial ways, the controversy and president's involvement, may be relevant to the 1980 contest's main questions about the incombent.

The president's best defense, legally, is the probably his worst offense, politically. As his aides tell it, the president's position comes down to this -- his judgment may have been poor, but his intentions were good. He wanted to make every effort to obtain the release of the hostages in Iran so his White House used brother Billy as an intermediary.

Good intentions, but poor judgment. That is precisely what Carter's critics have been saying about him throughout the campaign year. It is a refrain that the Republicans plan to carry into the fall.

But beyond that, there is the question of public trust. In 1976, when he ran against Gerald Ford, and in 1980, when he ran against Edward M. Kennedy, Jimmy Carter always scored highest in political polls in the area of trustworthiness.

Now that quality can be called into question too. If the president cannot maintain the image of personal integrity and trustworthiness, his chances of overcoming the 28-point lead that Ronald Reagan has in the polls are indeed slim.

Without the Billy Carter affair, the president's political problems would have been sizable but not insurmountable. Incumbent presidents have a way of coming back, aided by the fact that the American people are not fond of replacing an imperfect but known commodity with an unknown commodity. So it was that Truman came back against Dewey in 1948, and Gerald Ford came back to within a point of victory against Carter in 1976.

The difference in 1980 may be that Ronald Reagan will be able to co-opt Carter's corner on the political marketing or personal character and morality. cReagan, despite his history as a conservative hardliner, may wind up running in the fall as the candidate most seen as having the attributes that Carter was known for in 1976.

From a candidate's standpoint, there is never a good time to have a scandal; but from Carter's position, there probably could not have been a worse time to have the Billy Carter affair spill out into the public domain.

This was to have been the time when the Carter operators secured their lock on the Democratic convention that will be held next month in New York City. They had planned to assure that the convention would be in their firm control, to cement their hold on their margin of almost 300 more delegates than they needed to win the nomination.

Now, instead, a dump Carter effort is under way. It was begun, belatedly, a few weeks ago by a few Democrats known for their fund-raising but not for their public prominence. Their idea was to open the party convention by releasing delegates from their Carter and Kennedy commitments so they could nominate a third figure instead. The effort showed little signs of progress until Friday. Then, with the Billy Carter scandal adding to their disgruntlement, about 40 Democratic congressmen met to discuss the possibility of finding a third person to nominate for president.

The members of Congress were top-heavy with pro-Kennedy Democrats, who were anxious for any help they could get to change a convention rule binding the delegates to vote on the first ballot for the candidate they supported in their state primaries or caucuses.

But there were pro-Carter and non-committed Democrats there also, among them Rep. Thomas Downey (D-N.Y.), a young member who made a political career in his early twenties by testing the political winds and then trying to get out ahead of the sail -- at that time by supporting Carter. t

Many of those present cited the Billy Carter affair as one reason for their disenchantment with the president.

"We are not exactly having to restrain Democrats from leaping to defend the president," one of Carter's senior advisers noted as he reviewed the list of congressmen who attended. And that gets to one other problem that Carter faces in the controversy over his involvement with his brother's Libyan problems. Jimmy Carter never bothered to cultivate close friendships with members of Congress. And now, staunch Carter friends and defenders in Congress are virtually unknown.

The effort to open the Democratic convention and prevent Carter from winning the nomination is still a longshot, at best. But the fact that it has at least apparently begun to grow -- at the time that the White House normally would have nipped it in the bud -- may well be an indication of difficult times ahead for the Carter convention strategists.

Still, the president does have some advisers who remain sanguine about their prospects for keeping the convention in check and the nomination secured. Among them, according to several sources, is Hamilton Jordan.

The former presidential chief of staff, who has left the White House to become the president's chief campaign strategist, told several other top-level advisers Thursday afternoon that he was still convinced his forces would easily win the crucial convention vote to bind the delegates for the first ballot, and that Carter would easily win renomination.

The Carter officials say that their own headcount shows a "worst case" slippage of fewer than 100 delegates from Carter's margin of almost 300, and that this would be offset by some 100 to 150 Kennedy delegates who they believe will vote to bind the delegates for one ballot.

But the Billy Carter affair carries with it political damage that will last far beyond the convention -- and the Carter advisers bemoan the fact that Reagan is already benefiting from its impact.

"If it wasn't for this Billy thing, the media would have all gone back after the Republicn convention into analyzing and probing that crazy co-presidency thing," said one Carter strategist, referring to Reagan's efforts to convince Ford to be his running mate. "The ford people's version of it and the Reagan people's version of it would have been the best story around. But, well, we've sort of taken care of that, I'm afraid."

They did indeed. On Tuesday, the White House made public a white paper that purported to be a full explanation, but which it knew was in fact not complete. It then proceeded to serve up revelations like Carter's Little Liver Pills, one a day.

Initially, the White House announced that national security adviser Zbigniew Brzezinski had used the president's brother to set up a meeting with a Libyan official for help in freeing the U.S. hostages in Iran -- even though Billy Carter was well known to be under investigation at the time for having failed to register as a foreign agent. (Brzezinski, it was said, could not remember what prompted him to do this.)

Then it was said that the president had also held a later meeting with the same Libyan official. The next revelation was that Rosalynn Carter had telephoned Billy to ask his help . . . and then the disclosure that in fact, Rosalynn had probably suggested it to her husband in the first place, and that it was the president who had instructed Brzezinski to enlist Billy's services.

And finally (or at least, most recently) Attorney General Benjamin R. Civiletti changed his story to say that he had indeed discussed the Billy Carter case once with the president despite his previous statements to the contrary. Civiletti's admission came only after the president came across a written record of the conversation, one that presumably could have been subpoenaed by the Senate subcommittee investigating the affair.

All of this has so far brought the president political damage, but not legal culpability. The central facts are that the Libyans were anxious to have some high-level access to the administration, that -- to free the Iranian hostages -- the president used his brother as an intermediary with the Libyans, and that after that, Billy Carter received $220,000 from the Libyans.

"It raises questions of prudence and judgment -- questions of errors of judgment, but not questions of crime," said one Carter adviser. "So people may wind up saying, okay, he's had some poor judgment and he had some bums around him. Hopefully, that's all."

But politically, for Jimmy Carter, that may be damage enough. The Billy Carter affair is no Watergate. But it was Carter himself who helped set the standard for the post-Watergate era in 1976, when, in mid-campaign, he said he would fire then-FBI Director Clarence M. Kelley because he had allowed FBI carpenters to build some valances worth $300 at his home. The FBI director, explained candidate Carter "ought to be purer than Caesar's wife."

EPILOGUE: The Carter man is one of the president's most senior advisers, one of his most ardent defenders and one of his most loyal advocates. aHe still is that, but the past week drained him of most of his optimism. He is talking like a man who is of a mind to start writing his memoirs.

"Well, if we get eight years in office, fine -- but if it is only going to be four years, that's okay," he says. "I came to this job hoping to work for Jimmy Carter -- a fine man -- but also hoping to accomplish some important things for my country. I think we did.

"After this, well I hope we can come back . . . but I just don't know."