Hope for the last mess hall of Camelot! After 18 months in the streets, an old Washington institution called the Federal City Club is attempting a takeover of an even older Washington institution: the Sans Souci.

"we haven't got a deal yet," warned a cautious Charles Bartlett, the club's president. "we're working to try to get started on something."

Yesterday, about 150 members of this club born during the Kennedy years approved a preliminary deal proposing it rent the Sans Souci at lunchtime. This means that the fashionable Sans Souci -- a downtown restaurant once haunted by presidents and others atop Washington's pecking order -- would become a private club at lunch, open only to Federal City members and their guests. Dinner would still be open to the public.

The deal, according to one Federal City Club member, proposes that the club generate $360,000 worth of lunchtime business for the restaurant each year. So far, the restaurant management hasn't agreed to anything. But it's interested.

We're talking, that's all," said Bernie Gorland, the Sans Souci owner who first heard the $360,000 figure when a reporter called him last evening.

"that's not bad," he said. "sounds like a nice, round figure.

The 570-member Federal City Club (300 more have eating rights) was born in 1964, shucking the traditional men's-club racial bans. It was ensconed in a first-floor corner of the Sheraton-Carlton Hotel until March 1979, when the hotel management announced it was giving the club's space to a bunch of trade association representatives who looked more profitable.

Founding members of the club include columnist Carl Rowan, World Bank President Robert Mcnamara and former Treasury secretary C. Douglas Dillion.