Just when everyone thought the party was over, the band struck a new introduction, a cake materialized from nowhere and Diana Ross was singing "Happy Birthday to You" with a disco beat. It was a few days early, but Jimmy Carter (who will be 54 on Sunday) did not complain, and neither did the 1,400 Democrats in the International Ballroom of the Washington Hilton Wednesday night.

The atmosphere at the Democratic National Committee's $1,000-a-plate dinner was very much like a post-Camp David junior prom. Anyone who had thought the Carter image was fading a few weeks ago seemed to have magically forgotten it last night.

"You can't win an election on foreign policy," said one guest in a waiting line at the cocktail party before the dinner.

"Israel isn't foreign," answered another.

In a roomful of diehard Democrats, none was more enthusiastic than Averell Harriman, the granddaddy of them all. "There's never been any doubt in my mind that Carter would be reelected," said Harriman. "What he has done with the Middle East is one of the most extraordinary things any president in history has ever done."

"This is the first fund-raiser I have ever been to where there weren't any pickets outside," said Frank Moore, head of congressional liaison for the White House. "When I got here, I thought I was at the wrong hotel."

Besides a chance to sing along with Diana Ross (which they did much more heartily for "Happy birthday, Mr. President" than for an earlier sing-along number: "Reach out and touch somebody. Make this world a better place"), the guests received a deluxe meal, including filet mignon at approximately $100 an ounce. The wine flowed not freely but expensively: one bottle came with the meal at each table, but Jody Powell had to borrow money to pay $10 for a second at his talbe.

The dinner itself, according to DNC treasurer Evan Dobelle, pulled in $1.6 million. "Every seat here is paid for," he said, but other sources reported that 200 free meals were given to some administration staff people and party workers. "Did I pay $1,000?" said Sam Brown, head of ACTION. "Of course not: I'm employed by this administration."

"There's a law of physics that you can't sell out a political dinner," said another guest. "It's an ever-expanding entity."

Hamilton Jordan's arrival was greeted by a flood of television cameras. He was immediately collared by energy czar James Schlesinger for a tete-a-tete under the bright lights.

Asked how long he thought the president's newest "honeymoon" would last, Jordan replied. "You know things were never as bad for us politically as some people alleged they were. To some extent, this euphoria is exaggerated, and the fact is that Camp David did give us a big boost, but now with civil service reform and the energy bill - things like that - it's all starting to work like we always planned it would."

Asked about his personal problems with Jack Anderson and the Vesco case, Jordan declined to comment "because of my lawyer."

Carter spoke less about Camp David than almost anyone else at the dinner. Asked whether he thought the president should be exploiting this accomplishment more. Powell said, "I don't think it's the sort of thing that you should try to gild. It's there. I think people would want to seem him talk about the jobs to be done rather than spend time bragging on himself."

"Besides," Powell added with a flick of the cigar that is becoming his new trademark, "it's just not his style."

Never at a loss for words, presidential trade negotiator Bob Strauss was happy to talk about Carter's rebirth since Camp David. "When people start analyzing the performance of both the president and the Congress for 1978, they're going to find it's been an exceedingly worthwhile performance for both," he said. "Carter will get the credit he deserves - including Camp David but not only because of it."

A side attraction who seemed to draw more attention than the comedy team of Monteith and Rand onstage was Ellen Metsky, for whom Peter Bourne wrote one of his last prescriptions in the White House. "So you're Ellen Metsky," said one out-of-town guest in tones usually reserved for the likes of Greta Garbo.

Possibly the most eloquent speech in an evening that sparkled with superlatives was the one given to Vice President Mondale by Rep. Tip O'Neill (D-Mass.), Speaker of the House, "The United States was at peace around the world, we enjoyed steady employment, the stock market showed steady growth, inflation slowed down, and we passed five major pieces of legislation - all in the 13 days Walter Mondale was acting president of the United States," said O'Neill.

Never at a loss for words, Mondale told O'Neill: "Of all the introductions I've ever had, yours is the most recent." In turn, he introduced Carter, predictably, as "one of the world's great peacemakers."

Carter gave what sounded like the first speech of his reelection campaign, cataloguing accomplishments of the administration. He received a standing ovation when he announced that in Israel the Knesset had just approved the Camp David agreements by a margin of 85 to 19. There was less applause when he went on to talk about controlling the federal bureaucracy and stopping inflation, but cheers came on schedule for his conclusion: "We cannot rest on past achievements. We cannot be complacent. We have work to do, as Democrats and as Americans."

At the dinner were Chip and Caron Carter. Rosalynn Carter in a violet gown that was not extreme by contemporary standards but unusually low-cut for her, and the president's mother, Miss Lillian, whom he introduced with an anecdote. When he returned from Camp David, the president said, his mother greeted him with the key question about the meeting: "Jimmy, is Anwar Sadat already married?"