MIAMI BEACH, FLA. -- A dozen-year crusade to save the oceanside hotels of Miami Beach's Art Deco District has made these whimsical confections of the '30s the new darlings of young club hoppers and out-of-town investors.

The historic one-square-mile district on the south Atlantic side of the island holds more than 800 buildings, some 650 constructed in the art deco and Mediterranean revival styles popular from the late 1920s through the Great Depression.

The district, once called the American Riviera, has undergone a rapid renaissance from seedy slum to artsy hot spot. Nearly 50 hotels and apartment buildings have been substantially refurbished in the last decade, and renovation is underway for another 70.

Their cantilevered "eyebrow" shades, porthole windows and birthday cake pastels seem wonderfully cartoonish in an era of towering steel and glass.

The small, human scale of the buildings and the comic gaucherie that was intended to lift the gloom of the Depression have attracted a generation of new fans born long after the Jazz Age died.

They crowd into the mauve and mint-green cafe's on Saturday night and gather on the patios after work to soak up ocean breezes and rum drinks.

"The generation that grew up with them does not respond as favorably as the young. They associate it with the Depression, which was not a good time for them," says Richard Hoberman, president of the Miami Design Preservation League, formed in 1977 to save the deco buildings.

Left to decay when the postwar glamor faded from Miami Beach vacations, the hotels were boarded up or converted to tiny kitchenette apartments and filled with pensioners from the Northeast. The residents' average age soared to 67 in the late 1970s.

Gradually, the elderly middle class began leaving the beach for retirement villages that offered more social activities. The migration gained momentum during the 1980 boatlift, when the streets filled with Marielitos on the prowl.

The low rents that attracted poor immigrants also began to attract artists seeking inexpensive studio space. Developers began to snatch up the cheap beach-front property, whose prices have nearly tripled in the last 10 years.

Tony Goldman, owner of New York's Green Street Restaurant and the SoHo Kitchen and Bar, owns five Ocean Drive properties and is restoring them one by one. Don Meginley, lauded for housing restoration in Boston, bought the newly renovated Waldorf Towers with its popular basement jazz club for $1.6 million in 1986.

"People from out of town, they think we're the greatest bargain in the world," says Murray Gold, executive director of the Miami Beach Resort Hotel Association.

"This is all outside money coming in to this area, people who realize the value of the real estate, while we who live here are going by what it was 20 years ago."

About the same time the elderly stopped settling on the beach, an art deco revival swept the nation.

"The attraction is as much historic as it is form. Deco has become contemporary again. It's probably yuppie-related," says architect John Fullerton, whose company is renovating the old Bancroft Hotel on Collins Avenue and building a compatible new addition for the Day's Inn chain.

"Back when it was first being done, it was a very radical movement. Now the language of deco is very contemporary, born out of the postmodern movement."

Preservationists rediscovered the old hotels. Led by deco doyenne Barbara Capitman, a crusader with a knack for publicity, they were initially resented as outsiders trying to impede new development.

They succeeded in 1979 in placing the entire neighborhood on the National Register of Historic Places, making it the youngest architectural district to earn that distinction.

Slowly, the movement gained respect, and deco came to be regarded as the savior of South Beach.

Local ordinances now protect deco buildings with a six-month waiting period for demolition permits, giving preservationists a chance to buy the property, assuming the owner will sell. There is widespread backing for an even stronger ordinance that would ban demolition entirely for designated art deco buildings.

Zoning rules also impose strict guidelines for renovation and new construction, ensuring continuity in the district's low ziggurat skyline.

The beautifully restored beach also is a considerable part of the ambiance. Eaten away by decades of erosion, the white-sand beach was expanded 300 feet seaward during a $51 million reclamation project completed in 1984, and a wide wooden boardwalk now runs from the southern tip of the beach to 44th Street.

A variety of financial breaks also have provided incentive to fix up the properties. Federal income tax credits are available for up to 20 percent of the cost of rehabilitating historic buildings.

Local programs range from grants that cover up to one-third of the cost of rehabilitating commercial property to subsidies that bring interest rates down to 6 percent on loans for fixing up residential buildings.