A year ago, John Coleman was just John Who? Last night at a party that guests branded "A-1" or "Five Star," he was John Bennett Coleman, a bright new hope on the Washington party-giving scene.

"I think we have a new host in town," said Buffy Cafritz, one of many socialites enthralled by the prospect. "I haven't seen this kind of assemblage since (former Iranian Ambassador) Ardeshir Zahedi's parties. Everyone from New York is here."

Indeed they were, collected at Coleman's Fairfax Hotel under a steamy party tent set up in the parking lot. The main attractions were heiress Gloria Vanderbilt Cooper, who wore her grandmother's tablecloth-lace dress and arrived with singer Bobby Short, plus Princess Lee Radziwill, who wore a hot red dress and peeking black bra that Washingtonians whispered about. She arrived with architect John Warnecke.

Both couples used the "just good friends" line when asked repeatedly about romantic inclinations, although Warnecke, the architect for the renovated Fairfax, answered the question by mealy-mouthing a little bit.

"Hey, Lee, how the hell do I explain, ah, we, ah, have offices in New York and she has offices adjacent to mine," Warnecke said. "I'm an architect and a designer and she's an interior designer."

"We see each other professionally," said a cool Radziwill, who called off her marriage to Newton Cope earlier this year.

Radziwill, the sister of Jackie Kennedy Onassis, was surrounded by reporters and photographers, as Kennedys usually are. On the prospect of Sen. Edward Kennedy challenging President Carter, she said, "I just hope he will -- for everyone's sake."

Other luminaries included Interior Secretary Cecil Andrus, former sportscaster Phyllis George Brown, Wyatt Dickerson, British designer Zandra Rhodes, photographer James Auchincloss, newscaster Roger Mudd, White House Social Secretary Gretchen posten, Anna Chennault, former senator Edward Brooke, lots of senators and representatives, and actress Cloris Leachman, who arrived late.

"I need some hairspary," was the first thing she said when she walked in the door.

The official reason for the party was to celebrate the opening of the new Fairfax Hotel Bar. (Lots of pinepaneled walls, oriental carpets and prints of horses jumping over fences, but not quite as "sensuous," according to Gloria Vanderbilt Cooper, as Annabel's nightclub in London. She was dancing there with Short recently, but more on that later.)

The other official reasons for the party were, first, to allow Coleman to thank all the people who he says have been so nice to him, and second, to announce a pledge of 5 percent of his pretax hotel profits to Washington charities and local activities.

"Whenever I have an operation in whatever city, we try to make it part of the community," said this Chicago multimillionaire who claims he's given "several hundred thousand" to the Windy City in the "last several years."

As for the unofficial reason for the party, at least one guest had a theory: "John's trying to make an impact on Washington," said Tom Amory, an executive head-hunter and friend of Coleman from New York. "I think he's going about it in the right way."

"The right way" in this case meant champagne begofe dinner, crab claws, roast beef, chicken, salmon, asparagus, fruit and cheeses during dinner Bobby Short, the darling of cafe society, after dinner.

"I call this a saloon date," said Short, who is more used to playing piano at the White House when he's in Washington. "The East Room, you know, is hardly a saloon."

As it turned out, the tent was indeed like a saloon, full of inattentive listeners. "Do you hear me?" Short shouted to the crowd. Some called back yes, and some called back no, but as one guest put it: "The problem wasn't the sound, but the listeners" -- too busy talking politics or Hurricane Frederic to give Short the attention he is accustomed to.

Oh, and Bobby Short's dancing at Annabel's with Gloria Vanderbilt. "She's very good," he said. "I take her out a lot, and sometimes she takes me out. We're good friends."

While all this was going on under the hot tent in the parking lot, the regular guests at the Fairfax stayed in the air-conditioned lobby and watched the celebrities go by.

"I wasn't invited," said Morton Roberts, an astronomer from Charlottesville, Va., who is usually peering at the heavens. "But I'm having fun looking at the other kind of stars tonight."