Television has failed miserably as a medium or record, at least when it comes to American culture. TV contributes little enough of its own to the enrichment of national life; the least it could do would be to present and preserve the best works of our cultural heritage.

A few years ago there was talk here of starting a so-called national theater, like England's at the Kennedy Center. That would have been about as national as brunch in Georgetown. Television is the greatest potential national theater is world history, but most of its time is frittered away on frippery. It's a perpetual motion minutiae machine.

For several decades, ending in the 60s, the American musical theater was the envy of the world and a constant source of joy and invention. And yet very few classic, near-classic or even merely superior Broadway musicals have been made available to the national audience or permanently preserved on videotape.

George Gershwin's monumental "Porgy and Bess" has toured the world to fits of acclaim. It played the Soviet Union 30 years ago. But in the intervening 30 years, no one has been able to mount a full-scale production of it for television. TV executives have been too busy polluting the culture to worry about enhancing it.

Here and there are signs of hope, however -- the most immediate being a PBS "Great Performances" production of a certified American masterpiece: Frank Loesser's "The Most Happy Fella," to be seen tonight on Cannel 26 at 8 p.m. Giorgio Tozzi plays the Napa Valley vineyard owner and Sharon Daniels his reluctant mail-order bride in a performance taped last October at Detroit's Michigan Opera Theater.

"The Most Happy Fella" is a miracle that happened in the mind of a man. Loesser wrote the book, music and lyrics for the show -- a feat no current Broadway talent could equal -- and the score includes songs that became immediate and lasting hits, like "Big D" and "Standing on the Corner (Watching All the Girls Go by)."

More than that, it was, like "Porgy and Bess," a musical with operatic ambitions that were sometimes striking realized. The book may have aged, but the score has not, and it is sung in this taped production with all the grand passion and melodrama that Loesser intended.

The TV producers, David Griffiths and Lindsay Law, and the co-directors, Jack O'Briend and Emile Ardolino, caught the cadences and responsiveness of a live performance without the stodgy stagey look that haunts many PBS opera broadcasts. Some of Tozzi's singing was dubbed in later, but the performances of the two leads -- as the lonely old immigrant and the wistful young waitress -- are poignant and luminous.

As fine as it is, "The Most Happy Fella," invariably raises the question, why aren't there more such shows on TV? The backlog of Broadway plays and musicals -- and off-Broadway shows as well -- is so extensive and rich that public TV could start a weekly series of TV revivals and not run out of material for years.

Of course it would cost a fortune, but not every show requires mobs of sets and actors. One perky little item, "She Loves Me," by Bock and Harnick, is an intimate as breakfast in bed. In fact, public TV aired a production of it earlier this season, and thought it was buoyant and attractive, it was also, of all the insane things, another British import.

Perhaps the British are most fastindious custodians of American popular culture than Americans are. The circumstances are so nutty, they're unfathomable. It is criminial, for example, that with all the recordings and broadcasting technology at our disposal, such legendary, incomparable and spectacular performances as Zero Mostel's in "Fiddler on the Roof" were never recorded for posterity. Except for the orginial cast LP, Mostel's magical, incadescent moment now has vanished, forever out of reach.

In the '50s, there were TV adaptations of Broadway successes. Mary Martin flew into more homes as "Peter Pan" than had ever watched a single television show up to that time. A color, taped version was later made and some dumbhead at NBC accidentally erased it; if only TV executives were as concerned with yesterday's treasures as they are with tomorrow's profits.

It used to be that big-hit musicals inevitably went to Hollywood to be filmed for a national audience. They don't film the big musicals any more because there aren't any, or at least not many; Broadway is top-heavy with revivals as it languishes in moral and creative diphtheria. Television could do something to keep the American musical theater alive, but no one has shown much inclination.

Nearly 25 years ago, on an episode of "I Love Lucy," the Ricardos and the Mertzes went off one night to see a big current Broadway smash called "The Most Happy Fella." Well, it has taken a quarter of a century for American television viewers to join them in that audience.It's a shame, it's a pity, it's a disgrace; that's what it is.