The largest spend-the-night party most people can remember finishes off this morning with breakfast in bed for 217 overnight guests of the Regent of Washington, the capital's newest luxury hotel.

The $64 million hotel (Regent is a synonym for posh hotel everywhere) opened last night at 2350 M St. NW in the West End, Washington's newest place to be. As well as the houseguests, another 250 or so Cinderellas came to the black tie buffet.

Local politicians and socialites put aside their anxieties about tomorrow night's presidential debates to shake hands with shuttle celebrities who arrived from everywhere by airplane and Daimler limousine; and to critique the work of chef Andre Craeps and pastry chef Bernard DeCaix; the architecture by David Childs for Skidmore, Owings and Merrill, mixing an art nouveau glass canopy with columns, balconies and a dome; and the Charles Pfister interior design.

Princess Lee Radziwill, the New York designer, arrived looking elegant in a deeply cut black dress, despite the rigors of a National-bound plane that circled around before finally landing -- surprise -- at Dulles. "I'm here to see my mother as well," she said.

The debates were not, however, forgotten amid the caviar, salmon and the sugar model of the hotel. "I hope the candidates will do their best in the debate and the winner will serve with distinction," said publishing tycoon and former Ambassador Walter Annenberg, playing it safe. Charles Wick, head of USIA, noted he could come to the party "because I'm not debating."

On the other side of the house, William McCormick Blair, once a close friend of former Democratic candidate Adlai Stevenson, said, "I'm hopeful. I am a very strong Walter Mondale and Geraldine Ferraro supporter. I thought she did a good job in establishing herself as a public figure on the last debate."

Ron Walker, who was chairman of the Republican convention, said, "I hope the president knocks his socks off. But the incumbent does have to speak with a degree of caution." He introduced his wife Anne as the new deputy director of public affairs at Commerce.

Alejandro Orfila, the former Organization of American States secretary general, now with Gray & Co., said that "the debates are very important. It's democracy at work. No, I can't say who I'm supporting because I am a foreigner. I don't vote here."

Overnight guests checked into their rooms (which will cost paying guests $135 a night) or two presidential suites, five deluxe suites and 24 executive suites with one to three bedrooms ($275 to $1,200 a night). They were welcomed with Chinese tea, fruit and mineral water to keep them from starving till the 9 p.m. party.

They could loll about in the steeping and whirlpool baths in the Portuguese marble bathrooms (the bidets and water closets discreetly hidden in separate compartments), dry up with hair dryer and terry robe before festooning themselves in black tie and glitter, have their shoes shined (free) or perhaps even take the plunge in the pool or jump to conclusions in the exercise lounge. Those intent on business could convert their currency, dictate or turn over their floppy disks to the multilingual secretarial service.

The party covered all the hotel's public spaces, the Mayfair, the Mayflower Court and the Promenade opening onto the courtyard, the Mayfair Grill (with rare Canadian mahogany paneling), the Ballroom and Assembly.

Wallace F. Holladay and Wilhelmina (Billie) Holladay (not the singer) and J.W. Kaempfer Jr.,owners of the hotel, were there when the band, the Count Basie orchestra, started playing.

Wilhelmina Holladay, founder of the National Museum for Women's Art, said the museum will open here in the fall of 1986, and that Susan Casteras, now at theYale Center for British Studies, has been named acting director. Holladay and Kaempfer are Washington investment developers. Robert Burns (not the poet) and Georg Rafael (not the artist), respectively the president and executive vice president of the Regent International Hotels, which manages and markets the Regent, and general manager Jean V. Mestriner, a Dutchman with a recent stop in Kuala Lumpur, Malaysia, were also receiving congratulations.