A 900-foot stretch of clean sand has become the battleground for a spicy reterendum fight in this conservative city.

The contested area is called Black's Beach and it's tucked away in one of the city's poshest parts. La Jolla Farms. Accessible only by a precarious mishmarsh of manmade stairs and depressions caused by nature. Black's Beach is the only legally sanctioned swimsuit-optional zone in the country and San Diego's biggest tourist attraction.

On Sept. 20 voters here will decide whether it should remain that way.

Proposition D on the city's primary ballot asks whether nude sun bathing should be banned at all city beaches. But the question is targeted at Black's, the only beach among 15 her that allows it.

Curiously, the question is being raised at a time when Black's future seemed ensured, its presence almost accepted.

In May, on the third anniversary of its creation by the city council, Black's drew more than 43,000 visitors to help celebrate the occasion. In 1976, nearly a million people sunbathed there in various stages of undress.

On July 4, its crowd of 25,000 surpassed those at the world-famous San Diego Zoo, which logged 14,394, and Sea World, which drew 16,229. Crowds this summer at Black's Beach averaged 5,000 daily.

Its appeal was so well known that a Manhattan travel agency last year began offering tours here to nudists. The package deal include air fare, a room in a La Jolla motel, a car and instructions on how to negotiate the cliffs leading to Black's.

Meanwhile, the city council acted to make access to the beach easier. In the spring, it hired an architect to design and build a trail down the cliff face for about $120,000.

Things seemed to augar well for the beach. That is, until midsummer, when the nude sun bathing issue got hot.

Midsummer was when the city council, which only endorsed the swimsuit-optional zone by a one-vote margin originally, began gearing up for the scheduled September primary.

Opponents of Black's put pressure on council members up for re-election to prohibit nudity on the beach.

The opponents hastily coalesced into a group called "Save Our Beaches," later changed to "Save the beaches" to avodi the acronym. Proponents of the zone formed the "Nude Beaches Committee.

The two sides clashed on a variety of issues including crime at the beach, the cost to taxpayers of proposed improvements and whether public nudity should be sanctioned in a city that fancies itself as "America's finest."

Lee Hubbard, a city councilman and longtime opponent of the swimsuit-optional zone, said the area tarnished the city's good name.

Hubbard, council men Tom Gade and Gil Johnson, whose district includes the beach area, and assemblyman Jim Ellis became the major powers leading the fight for Proposition D. They are supported by some area churches.

Money to finance the campaign for the proposition has come mainly from residents of La Jolla Farms. Revenue to run the Nude Beaches Committee has been derived mainly from the sale of T-shirts featuring a nude couple and the words "Black's Beach, San Diego."

Nude Beaches Committee treasurer David Irving said the committee is comprised of people ranging in age from 20 to 60. He admitted, however that the vote of the young is needed to defeat Proposition D. They are more supportive of the beach than older residents, Irving said.

Irving said the 80 or so families living in the wealthy area found the beach are its biggest opponents.

"They had a very quiet, residential area," he said. "Since Black's has become widely known they see the same traffic and litter problems as the people in the city's other beach areas.

If Proposition D passes, Irving predicts the city will have a hard time enforcing it.

"One in six people in San Diego likes to go to the beach nude," he said. "A lot will continue to go even if the city says no." Bathers, he said, would be able to spot police officers a mile away.

He did not mention how, if the police were dressed - or undressed - like any bother bather.