THE NATIONAL THEATRE -- virtually left for dead after the opening of the Kennedy Center eight years ago, then restored to shaky health under the Center's paternal ministrations, then threatened anew by the designs of an ambitious neighbor, the National Press Club -- is preparing to reassert itself.

The theater is about to engage its own independent booking service, ending a five-year booking arrangement with the Kennedy Center.

The National's plans have been spurred by an agreement to be signed this Wednesday between the Pennsylvania Avenue Development Corporation and Quadrangle/Mariott, the group that is developing property on three sides of the National. This agreement calls for substantial renovation of the theater at Quadrangle/Marriott expense. The National will acquire a larger lobby, expanded restrooms, an elevator to the mezzanine and balcony levels, new and refurbished dressing-room space, rehearsal facilities, a stage-level loading dock and a new system of heating, air-conditioning and ventilation.

But what may be more important to the theater's future is the symbiotic promise of being nestled in among 50 to 75 new stores, half a dozen restaurants and sidewalk cafes, an 830-room hotel and a 760-space parking garage -- all within weather free reach through a covered arcade along E Street NW (soon to be redesignated a part of Pennsylvania Avenue).

Quadrangle already has begun work on an adjacent office building at 13th and E (or, in "new downtownspeak," 13th and Pennsylvania); and the ground under the main complex is supposed to be broken by mid 1983.

Robert Gladstone, president of Quadrangle Development, sees the National, Ford's and Warner Theaters forming a new downtown entertainment core, and he talks about package theater plans for guests at Marriott's new "flagship" hotel on the Quadrangle/Marriott site. "It's an exciting opportunity," he says.

"It's not the sort of a lease (for the National) that we expect to be economically profitable in the sense that a store is," says Gladstone. "But we are satisfied that the idea of a legitimate theater at this location is a sustainable one." The National will continue to pay a rent in the $100,000-a-year range, with periodic adjustments for inflation and tax increases.

To go with the new mood and look, National Theatre officials have adventurous -- if vague -- programming plans. They hope for a new subscription arrangement, involving either the National alone or a larger group of Washington theaters. "There has to be a subscription because otherwise you're dead out of water," says Maurice Tobin, chairman of the National's board.

And the National aims to graduate from a diet dominated by second string touring productions and return engagements to bolder fare. Last season, when the theater pulled in a record $6 million in gross ticket sales, "we had to have blockbusters," says Tobin. "We had to concentrate on keeping the wolves away, but nobody wants to just re-book shows. The fun thing is to get out and get the new, the imaginative."

A possible joint arrangement with a group of resident theaters -- Seattle's ACT, Los Angeles' Mark Taper, Minneapolis' Guthrie and New Haven's Longwharf -- is in the "just talking" Hase, according to Tobin. And the National has apparently decided to change its booking arrangement with the Kennedy Center, a relationship criticized by Sen. Bill Bradley (D-N.J.) for allegedly giving the center and chairman Roger Stevens a monopolistic hold on commerical theater in Washington.

Since the departure of "A Chorus Line" last spring, the National has not had an outstanding success. "Roger Stevens has been a real kingpin for us," says Tobin, but "we signed a Kennedy Center management contract with the understanding that any time we want, we can do business with others".

If Tobin's grand vision comes to pass, it will be a remarkable 10-year turnaround for the National and its environs.

In the early '70s, says Tobin, "People just didn't want to go to the National: It was a poor, distressed old lady.Instead, they could go to the Eisenhower and park their car in the basement and walk down those gorgeous mirrored allees. It's just very difficult to compete with that. The physical convenience of theater (at the Kennedy Center) was fantastic, and the competition left the National in the lurch."

At its worst moment, the National was $250,000 in debt, according to Tobin, and had to lease a water-cooler because it couldn't affort to buy one.

The Nederlander family had run the theater through 1972 and '73, but when they decided to throw in the towel, the National was taken over by a non-profit corporation that Stevens was instrumental in founding. At the same time, the National signed a booking agreement with the Kennedy Center, following the time-honored "if you can't beat 'em, join 'em" formula.

Since then, there have been encouraging ups and draining downs. The $250,000 was paid off in '75 and '76, followed by a $300,000 refurbishing (with hand-me-down carpets from the Kennedy Center). The musicals "Annie" and "A Chorus Line" enjoyed long and profitable runs. But the vacancy sign also has been posted periodically, as it was through most of the summer of 1979 (when the musical "If! If! If!" was abruptly pulled from Washington by its producers).

Compounding the National's injuries was the ultimate insult -- a plan proposed by the National Press Club and Atlanta developer John Portman to wipe the National off the map in the course of building a vast new hotel-office-retail complex. It was the National's organized resistance to this plan (with support from preservationists, politicians, the press and such theater folk as Carol Channing, who pleaded for the National's survival during "Hello, Dolly" curtain calls) that led to the PADC'S decision to reject the Press Club plan a year ago and sign a deal with Quadrangle/Marriott instead.

No one argues that the National should be saved for its purely architectural merits, but the teater's friends see its bland form as superbly functional. Actors extend that thoroughly exhausted word of highest praise for a theater -- "intimate." And acoustical engineers, according to Tobin, "come from all over the country and ask, 'How was this done?'"

Then there is the National's debated status as the oldest continually operating theater in the country. Tobin debunks a similar claim by Philadelphia's Walnut Street Theater on the grounds that it was briefly turned into a car barn during the Depression. The National, since its opening in 1835, has burned, collapsed, served time as a circus and a movie theater, and repeatedly stared down the threat of demolition.

And manager Richard Schneider insists that despite all these upheavals, there are at least a few bricks on the premises that have been there for the whole 144-year duration.