Not many class reunions are held eight months after the old gang busted up, but not many classes are $600,000 in the hole.

That debt from Jimmy Carter's campaign for the Democratic presidential nomination last year helps explain the timing of the Carter-Mondale reunion Saturday, the first of what former president Carter hopes will be annual events.

On a gorgeous Indian summer evening, 150 Carter supporters, former administration staffers and Cabinet officers gathered around a catered buffet and bar on the lawn of the Swan House mansion in north Atlanta to kiss, hug, chat and fork over what was expected to total $100,000.

For their part, Carter and wife Rosalynn were such diligent greeters that they forgot to eat. They mixed amiably with such old acquaintances as former attorney general Benjamin Civiletti, former U.N. ambassador Andrew Young, now the front-runner in the Atlanta mayor's race, and former attorney general Griffin Bell, now practicing law in Atlanta.

Serenaded by a string quartet playing as much Scott Joplin as Brahms, the well-dressed guests munched on Swedish meatballs and listened to Carter's anecdotes about his recent trip to China, where he found the press "refreshingly positive."

Former vice president Walter F. Mondale later referred to his old boss' quip to China's vice chairman, Deng Xiaoping, that "If you had been my running mate, we would have won again."

The question of why Carter got in Dutch with the voters last year was touched on frequently. Carter's press secretary Jody Powell lamented, "The only thing we got out of Washington was the boot." And with only eight months of out-of-office events to catch up on, the not-always-loyal opposition devoted much conversation to talking down the Reagan administration and revering Carter's.

"We're watching what's happening in Washington; it's incredible," said Rosalynn Carter, grimacing.

"To see a lot of our programs dismantled is disturbing," the former president allowed. "There's no doubt in my mind that this administration has made some very serious mistakes."

For instance, said former treasury secretary G. William Miller, between autographing dollar bills for guests: "Reagan's tax cuts are too soon and too broad. Let's hope Reaganomics will be successful, but it looks like we have real trouble."

"I don't think that Reagan's numbers add up. It's like borrowing money to pay a dividend," chimed in Bert Lance, Carter's ousted head of the Office of Management and Budget, now cooling his heels in Calhoun, Ga., where he once again has controlling interests in Calhoun First National Bank.

"The thing that will stand out about the Carter administration is its compassionate concern for human beings," Lance continued. "I don't see any evidence of that in the Reagan administration."

Many observers said they feel there's little evidence left of the Carter years in Washington, an attitude alluded to when Carter introduced Mondale as "one of my old friends and associates who's still not ashamed to admit it." Carter's Georgian supporters are well aware of the snickers heard from some quarters when Egyptian President Anwar Sadat recently suggested that "Jimmy Carter has left his fingerprints on the history of his age." But Carter loyalists are confident that history will not regard the late 1970s as the Lost Quadrennium.

"When the bottom line is in on this Carter's administration," said Powell, "it will be that he was a president who did what he thought was good for the long-term good, without regard to political cost."

Apparently, many of the exiles have adopted a philosophical view of last November's shellacking. Former White House aide Jack Watson, expected to be a candidate for Georgia governor, offered this parable: "I've always loved the story of Cincinnatus, the great Roman conqueror who returned to his farm. That made a lot of sense back then and it makes a lot of sense now. People shouldn't have to make politics a lifetime career."

The ex-president himself said he'd made his peace with defeat. "The hurt's gone," Carter said. "I've enjoyed not being president. Plains has been better than we anticipated."

For that matter, he said, "We enjoyed the life in Washington. Amy was in public school there, and I ran and played softball in the parks. We tried to be a part of Washington life, and we succeeded." Next month he and Rosalynn will return to the capital for a dinner honoring Averell Harriman, their first trip to Washington since leaving the White House.

Does Carter feel, as many do in the southern provinces, that the Washington political and journalistic establishments were out to humble the conquering Georgians?

"It would have been hard for a Georgian up there to avoid close scrutiny," he said. "What was done to Bert Lance, Hamilton Jordan, Peter Bourne, my brother, myself was of great concern to us. But we didn't let it get us down, and I think it turned out well."

Hamilton Jordan, who held forth close by the meatballs much of the night, agreed that at times it seemed the Georgians were mugged by the media. But the former White House chief of staff -- his face pink as a rare Porterhouse -- said he bore no resentment. For now, he lives in a home in Atlanta, where he was host to many in the reunion bunch at a private brunch yesterday. He's lecturing at Emory University, writing a book and awaiting the day America will wax nostalgic over killer rabbits, amaretto and cream and other such Carter administration legends.

As the reception wound down, the still-famished Carters were presented with doggie bags filled with fruit and other goodies. Presently, a photographer asked them to pose, and they did so, booty in hand.