THE WASHINGTON Opera is in danger of losing the fine momentum it acquired when George London took over its general direction several seasons ago. London moved steadily to improve those elements in the company's productions that are essential if opera is to combine artistic and economic success: repertoire, casting, staging and a healthy box office.

London was aware of the history of the Opera Society, of its standard repertoire, its notable achievements in bringing Washington major premieres of operas by Ginastera and Delius, and its strong record in Mozart, Verdi, Debussy, Strauss and other operas in which London himself had often appeared. He also learned that seasons prior to his appointment had been unsatisfactory to many, emphasizing as they did operas of values that are, to put it mildly, debatable.

To the internal matters he set about to change, London has added his strong views about the necessity of finding and encouraging the best young American singers, a program he supported both in his years as executive director of the National Opera Institute and in his seasons with the Washington Opera.

Unfortunately fate has been cruel to George London in the last two years, plaguing him with severe vocal problems, and last July, while he was coaching in Europe, striking him with a heart attack from which he is slowly recovering.

But an opera company needs firm direction at the top as its day-to-day operations and longterm program planning are kept vigorous and alert. London is unable to give the Washington Opera this kind of direction. His absence from this year's productions, even though he planned many details last year, has been sorely felt. It is not mere dreaming to believe that if London had been available, the "Magic Flute" that opened the season would have taken on some of the sparkle, the animation, the sense of style that it so grievously lacked.

One of the most intelligent, dramatic figures on the international opera scene in the '50s and '60s, London is not the type to permit lackluster direction to stifle productions given under his aegis.

Without his presence, the company is drifting toward a crucial failure of informed planning and successful artistic attainment. The Washington Opera needs - and immediately - a new general director. For many reasons, an ideal candidate is Sarah Caldwell, the eminently successful five-star general of the Opera Company of Boston.

For 20 years Caldwell has presented opera in Boston on a musical and dramatic level that has won her acclaim throughout the opera world. Sheknows, from long experience, every problem of casting, of working with the world's greatest singers and its most promising young newcomers.

She knows the pains of scrimping for the last penny, and all the headaches of raising an annual budget. She has given some of the most important operatic premieres in this country's history, including those of Luigi Nono's "Intolleranza," Berlioz' "Troyens," Roger Sessions' "Montezuma," and many more. She has offered her Boston fans, who are rabid in her praise, Shirley Verrett and Jon Vickers, Tatiana Troyanos, Beverly Sills and George London, and at the same time, young artists whose names have become famous at home and abroad.

Her staging and conducting of Prokofiev's "War and Peace" at Wolf Trap in the summer of 1974 was one of the great triumps on that stage.

Next week Caldwell returns for her second conducting assignment at the Metropolitan Opera, this time to conduct the first of five performances of "L'Elisir d'Amore." It follows by some years her unforgettable work as music director of the excellent, if short-lived American National Opera Company.

Caldwell, whose fame spread like brushfire last year when she appeared on the cover of Time magazine, is now regularly conducting major orchestras across this country. But it is interesting to conjecture about her response if the Washington Opera were to invite her to take charge.

Her Boston season does not open until after the first of the year, although under her direction Opera New England presents performances from Connecticut to Maine in the fall. These are usually conducted by a young Caldwell protege.

Another reason Caldwell could handle Boston and Washington is that both companies present about the same number of operas in a season. It would not be difficult to dovetail the two schedules. Furthermore, in a time when cooperation between opera companies is becoming more and more appreciated for artistic and economic reasons, a Washington-Boston alliance could be useful to both cities.

If Boston enjoys "Benvenuto Cellini" by Berlioz, with Jon Vickers singing the title role, surely Washington would too. The great Canadian tenor also sang Florestan in "Fidelio" for Caldwell. The very fact that an opera like "Cellini," which is not sung frequently, could be given in two cities rather than one would be a powerful argument in persuading singers to learn its roles.

Still another factor makes the idea of a Caldwell regime in Washington quite special: In March, Robert Shaw, conductor of the Atlanta Symphony, will make his operatic debut, conducting performances of Berlioz' "The Damnation of Faust" in Boston, where he will also, with his singular gifts, give special training to the chorus.

Now what city is midway between Atlanta and Boston and in an ideal position to profit from any operatic activcties in either? (Never mind where the president of the United States comes from, he does happen to be an old admirer of Shaw's musicmaking).

Sarah Caldwell has proven, in the same two decades in which Carol Fox has demonstrated the fact in Chicago, that women who know their business can be, in quite different ways, unsurpassed as heads of opera companies. Washington needs new, experienced, strong general management of its opera company. Sarah Caldwell has worked operatic wonders in Boston. She could work them here.

If Caldwell is unavailable or not interested in expanding her kingdom, there are other qualified experts in opera direction who would welcome the chance to work in Washington. This city has, for many years, shown its interest in and devotion to a local company. In George London's enforced absence it is essential that someone be given the active direction of the company for which he has done so much. If his health permits his return later on, it would not be a difficult matter to take advantage of his special talents.