What a collective case of on-top-of-the-worlditis there was at a party for the American Enterprise Institute Last night. "An apex," assessed former Federal Reserve Board chairman and present AEI member Arthur Burns. "And on the way straight to heaven."

The place to be bombarded with Ronald Reagan's economic policy this weekend was not, as might be expected, a solemn White House briefing. It was actually at this AEI cocktail party in the Hay-Adams Hotel, a site where there seemed to be more administration wares than there are proposed cuts in the budget.

"Unprecedented! Unprecedented!" said Murray Weidenbaum, chairman of the Council of Economic Advisers and a man who, at least last night, sounded like an impassioned devotee as he talked of reaction to the cuts. "It used to be that people would say, 'Don't cut my budget.' Now it's 'We know you're going to cut my program -- but don't just pick on me.'"

Leonard Garment, former counsel to the Nixon White House, strolled by. "If you need sanctuary," he said to Weidenbaum, "give us a call." Garment then walked toward Brent Scowcroft, formerly of Nixon's National Security Council, but stopped to chat with a member of the press.

Scowcroft eyed both the press person and Garment, then turned to his former Colleague and pleasantly asked: "Are you leaking again?"

Officially, the party was for AEI, the conservative think tank, to honor members appointed to the Reagan administration. At last count that's 16 1/2, the half being theologian Michael Novak. He's with the new administration only temporarily.

But even without him, the appointees -- including Weidenbaum, U.N. Representative Jeane J. Kirkpatrick, presidential assistant David Gergen and International Monetary Fund Executive Director Richard Erb -- still represent more than 10 percent of AEI's current members. And what that means, besides slots for some former Carter administration members that AEI says it will hire, is more influence.

"I'm just kind of basking in their reflected glory," said Ed Fuller, the president of the conservative Heritage Foundation, who had dropped by to enjoy some of the victor's spoils and hors d'oeuvres.

AEI president Bill Baroody Jr. was slightly more cautious about a now-powerful group that in the old days was referred to as "rag-tag" and "hole-in-the-wall."

"Oh," he said modestly, "I think our challenges are ahead of us."

Like what? "Well," he said, "like helping to produce effective policy formulas that are going to revitalize America." He was more modest the first time.

The two-hour party was absolutely stuffed, the state of most Republican get-togethers these days. The AEI members who are now Reagan appointees could be identified by red carnations and the small but eager crowds that formed around them.

It was in these clusters that you could hear snippets of Reagan's economic package, the specifics of which he will broadcast on Wednesday. Immediately afterward, his troops are to begin a public relations blizzard in support of the program. Bill Baroody's brother is in the P.R. firm that will be handling the blitz.

"It's a wonderful client," said Joe Baroody of Wagner & Baroody, the veteran Washington Firm that has served three Republican administrations and lots of GOP candidates, "but it's probably a loss leader." He laughed. "But we'd do it anyway. It's wonderful."

Besides conservatives, the party was filled with Washington journalists, many of whom are liberals. In a Republican administration, they find they must do a good deal of catch-up schmoozing with the new sources. And they certainly were last night.

"This is probably one of the most important groups in the city right now, don't you think?" said Frank Mankiewicz, the National Public Radio president and past manager of former Sen. George McGovern's presidential campaign.

"This is a gold mine," said another reporter.

Among those to whom reporters were being nice was John Rogers, an AEI special assistant who's now at the White House as a special assistant. aHe's 25.

"How old?" asked an astonished Deborah Styles, a free-lance editor and wife of Edward Styles, the AEI publications director.

"Ah . . . 25," said Rogers.

"Oh, my God," responded Styles.

"We'd like to keep that a secret," said Rogers, who might have looked 19 if he hadn't been wearing grownup business attire. "A state secret."

Nonetheless, the youngest person in the room was undoubtedly Christopher Gergen, 10. He's the son of David Gergen, the presidential assistant.

The young Gergen was subsequently asked if he is a Republican.

"I haven't made a decision," he replied. "But I might be like my dad -- start as a Democrat and turn out a Republican."

"Oh?

"Way back when," replied the elder Gergen. "In North Carolina, when I was working for civil rights. I don't think I even met a Republican until I was 21."