Marilyn and Harry Lewis have sunk about $100,000 into their plan to start a Hamburger Hamlet on M Street in Georgetown - but the first hamburger has yet to hit the grill.

Instead, the California couple, who launched the nationwide restaurant chain 28 years ago with $3,500 and Marilyn Lewis's grandmother's "secret lobster bisque," has become part of the longest and fiercest liquor license hearings in city history.

The anti-Hamburger Hamlet forces, a coalition of influential Georgetown residents and merchants, say the number of bars and restaurants in their neighborhood has reached a "saturation point," generating unbearable levels of traffic, litter and noise. They want the city to declare a moratorium on all new liquor licenses in Georgetown.

The Lewises, who apparently had no idea that obtaining a liquor license would be more than a formality when they leased the M Street location a year ago, counter that Hamburger Hamlets are "family restaurants" of a sort Georgetown lacks.

"It's not a bar, it's not a fancy restaurant, it's not a carry-out," said Georgetown lawyer James C. Conner, one of the pro-Hamlet witnesses who recently led off three days of hearings before the D.C. Alcoholic Beverage Control Board. Conners and others told the board that Georgetown, for all its liquor licenses, has few restaurants suitable for adults with children.

The ABC Board is expected to issue a decision soon.

The campaign against the new restaurant has been spearheaded by the Citizens Association of Georgetown, one of the best-organized and best-financed neighborhood organizations in the city. Last year, after a series of mostly successful efforts to defeat liquor license applications, the association won two major victories by convincing the ABC Board to deny applications from La Potagerie, a French restaurant, and from the Pierce Street Annex, a proposed "California-style" cafe, both on M Street.

Despite those decisions, the ABC Board has repeatedly told advocates of a moratorium to take their case to the City Council. Until a moratorium is enacted, acting board chairman James W. Hill has said, "We have to decide each case on an individual basis."

At the Hamburger Hamlet bearing, Hill reiterated that view, but for the first time said the board might support the moratorium idea. "If that's what Georgetown wants, this board would like to help you get it," he said.

There is a widespread belief among some attorneys, neighborhood activities , restaurant owners and others involved in the Georgetown liquor license wars that a moratorium bill will at least be proposed, if not enacted, soon after city elections this fall.

But the councilman who would probably have to introduce such legislation has said it is about the furthest thing from his mind. "When I'm ready to address liquor, I'll address liquor," said John A. Wilson (D-Ward 2), chairman of the Council Public Services and Consumer Affairs Committee. ". . . My staff is working on it. I haven't even looked at what they've done."

Asked to indicate his current thinking on the question, Wilson said, "I don't have the slightest idea what my thinking is." Nor would he predict when a bill might be presented. "I don't work on schedules, I work on impulses."

Hamburger Hamlets, Inc., has been a publicly owned corporation since 1958, although the Lewises still hold more than 70 percent of the stock. The company operates 20 restaurants, including 17 in Southern California, one in Chicago, one an old Georgetown Road in Bethesda and one at Wisconsin Avenue and Jennifer Street NW. The company reported total earnings of about $17 million in 1977.

An official at the firm's Beverly Hills headquarters said the chain serves an assortment of soups, salads, crepes and other fare other than hamburgers. The average customer's check comes to about $5.50, the official said.

"They're very quiet places either for families or for rather straight couples," said Michael Nussbaum, one of four attorneys who presented Hamburger Hamlet's case before the ABC Board.

Marilyn Lewis, who has risen from cook to chairman of the board, generally does business from alongside her swimming pool in Beverly Hills, according to recent Fortune Magazine profile. But she has been living in the Washington area lately, planning the M Street outlet.

Last year she taught a course at American University called "An American Dream - Owning Your Own Business," and, as she told the ABC board, she has established "Hamburger Hamlet Scholarships" at AU and Howard University.

Harry Lewis, the company president, is a former actor who played a gunman in "Key Largo" with Humphrey Bogart and Lauren Bacall, and recently appeared in the TV mini-series "The Moneychangers."

The Lewises told the ABC Board they expected to spent about $1 million on the Georgetown restaurant, assuming their liquor license is approved. In the meantime, Hamburger Hamlets is paying $8,000 a month to rent two adjacent buildings at 3123 and 3125 M St. NW, according to the Lewises' testimony.

The two buildings, formerly occupied by Arpad's Antiques and Tribby's Jewelry Co., are in the middle of one of the city's liveliest retail corridors. Restaurants and bars within a two-block radius include Mr. Smith's, the Publick House, Nathan's, the Port of Georgetown, Le Steak and the Cafe de Paris.

Anti-Hamlet witnesses, some of them residents of the immediately surrounding streets, emphasized questions of noise, traffic and litter.

"I can count on being waked up at least once every Friday and Saturday night," said Ann Weeks, of 1243 33rd St. NW.

"I had to move my child's bedroom from the front of the house to the back of the house to escape the noise," said Caren Pauley, who lives at 1236 33rd St. Pauley is a member of the local Advisory Neighborhood Commission (3A), which has opposed the Hamburger Hamlet application. "We have over 25 places that I can name that serve humburgers."

An entire day of testimony was consumed by a long, bitter debate over Georgetown traffic, with Robert L. Morris, an expert witness called by Hamburger Hamlets, insisting that the situation was not as bad as residents portrayed it. Even at peak hours, said Morris, traffic through Wisconsin Avenue and M Street NW is never worse than "service level B" - on a scale of "A", the lightest traffic, to "F", the heaviest.

This brought mutters of contempt from opposition witnesses, who described Georgetown as a monumental traffic jam on Friday and Saturday nights, and derisive questions from attorney Courts Oulahan, representing the Citizens Association of Georgetown, who dismissed Morris as a "professional witness" for developers.