For as long as anybody can remember, politicians have been promising to clean up one of New York's meanest streets - West 42d Street between 6th and 8th avenues - but there is little to show for it.

Mayors and mayoral candidates, posing sometimes with the symbolic broom, periodically have predicted the rebirth of the once great crosstown thoroughfare, whose reservoir of theatrical talent gave rise to the Ziefeld Follies and whose charm was immortalized in song by George M. Cohan.

Most recently, Mayor Abraham D. Beame invaded the neighborhood with a retinue of newspaper and television cameras, hoping to embarras some unimpressed pornography merchants, and Gov. Hugh L. Carey approved a law banning topless dancing in bars, calling it a "bill against dirt."

Yet the street, in some blocks so degenerate that even prostitutes have contemptuously written it off, remains a wasteland - useful only as a drug drop or a sanctuary for derelicts and other flotsam of society.

To be sure, crudely painted marquees advertising live sex shows and coin-operated "peep" movies suggest that commerce of a sort does flourish, making the proprietors even more intransigent to demands that they leave.

As one real estate man said of 42d Street, "she's an ugly cow, but she gives a lot of milk.?

Now, where the resources of the nation's biggest city government have failed, a small, non-profit redevelopment corporation operating with little fanfare out of sparsely furnished offices in a mostly empty building has taken on the Herculean task of restoring 42d Street to the greatness that Gertrude Lawrence, Bea Lillie and Will Rogers gave it.

Since its formation only 16 months ago, the 42d Street Redevelopment Corp. has put together the most ambitious plan yet devised to salvage not only the seamiest West Side core section of the street, but its entire length from the East River to the Hudson.

And as if to offer proof that it is not dreaming the impossible dream, the corporation - whose board members include Jacqueline Onassis and former Port Authority chairman William Ronan - will soon begin renovations that will turn a row of crumbling tenements and former pornography shops into a row of five off-broadway theaters.

By October, theater companies will have moved into the buildings, creating a solid row of five 99-seat Equity Showcase theaters plus 10 floors of rehearsal and office space.

The group also plans to expand and dress up the face of a 70-year-old butcher shop in the same block, adding a steak restaurant and creating a miniature Shubert Alley leading to a landscaped intermission garden behind the new theaters.

Financing for the theater row, backed jointly with the New York State Urban Development Corp., a government renewal agency, is complete, according to the 42d Street Development Corp. president, Frederic Papert.

While receiving little aid form an already financially strapped city government, the corporation has relied on private donations, grants from foundations and corporations, bank loans and sometimes forebearance agreements on mortgages of long-abandoned and essentially worthless buildings.

Also, as a local non-profit development corporation, Papert's group has been able to buy property from the city without competitive bidding, or has donated property of its own to the city and leased it back tax-free.

West Side Properties Inc., a subsidiary of Papert's group, already owns 14 buildings across the street from the Manhattan Plaza apartments, a new high-rise tower whose subsidized tenants include 4,000 people in the performing arts.

The theater row, along with the Manhattan Plaza complex, will be an "incubator for performing arts companies," according to Papert, who estimated that there are twice as many amateur companies in New York as there is performing space.

Theater row, according to Papert, is only the beginning.

The corporation bought a building on 42d at 7th Avenue, drove out through condemnation proceedings the owner of the city's biggest "peep" show parlor, and installed in its place a 24-hour police substation. A number of merchants on the street, according to Parpert, had agreed that if police presence were increased they would contribute capital to sidewalk improvement.

The fledgling corporation has not stopped there.

It acquired - free from the Port Authority - the mostly vacant West Side airlines terminal, which it plans to convert into a 400-seat dance theater. And under a grant from the National Endowment for the Arts it is studying a plan to convert a former Army Reserve training center between 10th and 11th avenues into a center for teaching and performance arts companies.

Its West Side subsidiary has also signed an agreement to acquire a sprawling truck garage at 11th Avenue, which it plans to convert into an equestrian center with stables for scores of horses. The city's mounted police squadron would move back to Manhattan from its new home in Queens, and the center would provide additional stable facilities for private horse-owners. It would also have an indoor ring large enough for three-man polo.

Also, the corporation hopes to develop a farmer's market on the Hudson River shore, covered with an air-pressure-supported plastic bubble. It has applied for mass transportation subsidies to develop a network of street cars or moving sidewalks to move people from midtown to the market.

Nobody - least of all the 42d Street Development Corp. - is guaranteeing that all of the planswill be fulfilled.

But Parpert - who has shied from publicity out of a professed fear of seeing pie-in-the-sky plans fall short - says his group will take one step at a time from the base of the new theater row.

"This particular street is physically and spiritually dead. The people are walking dead people, and anything we do in this neighborhood is going to help," he says.

"The idea is to make some improvement around here, and link it to the easterly portion of 42d Street, which contains some of the best attractions in the world," he added. These include the United Nations, the Chrysler Building ("the best art deco attraction anywhere"), the New York public library, City University and the Daily News building.

"We need to renew the westerly blocks, upgrade the middle blocks and link them all to the east end, creating a grand river-to-river boulevard. It will attract business and people and make this old street good again."

But Papert and other supporters are insisting that the formidable task be undertaken gently.

"Most good works suffer from under-capitalization. We're trying to keep these projects low capital intensive. This is essentially a themed commercial renewal program, and there's no reason that it should cost huge amounts of money."

The economics, Papert said, are simple. "If the farmer's market were here, we'd have no trouble getting the mass transit money. It the mass transit were here, we'd have no trouble getting the capital for the farmer's market. The trick is to get either one of them."

However, once enough projects are operating successfully, the corporation's budget will be covered by rental income, Pappert reasoned, and the development of the street will be assured.