The brilliance, and the wane, of de Kooning's artistic genius

Dramatic proof of what an extraordinary artist Willem de Kooning has been is on view at the Whitney Museum here. ''Willem de Kooning: Painting and Sculpture'' and ''The Drawings of Willem de Kooning'' jointly constitute the most important retrospective of this artist's work to date. Included are over 250 paintings, sculptures, and drawings dating from his student work of the 1920 s to the large abstractions executed only a year or so ago.

Viewing this exhibition is both a tremendously exhilarating experience and a slightly melancholy one, for it both totally vindicates his reputation as one of the most original and dynamic painters of the 1940-65 period and dramatically underscores the fact that the quality of his work has been in decline ever since.

Someone involved with this exhibition should have done the artist the favor of limiting his recent works on view to the half-dozen or so paintings that still bear evidence of his original painterly flair. It would have improved the show considerably, and would have given a fairer accounting of de Kooning's true quality and importance.

A judicious pruning of most of his weaker recent work would have permitted a clearer focus on his major contributions to contemporary American art. What a painter this man was when he was ''on top,'' when Abstract Expressionism was in its formative stage, and then when it moved on to consolidate its gains! From roughly 1950 to 1960, it was de Kooning, as much as anyone, who led the way, whose work was the center of attention in any large exhibition, and to whom younger artists looked for inspiration and guidance.

From New York to San Francisco, de Kooning was acknowledged to be in the forefront of the great push to make painting magnificent once again, to give it back its pride lost somewhere back in the early 1930s in Paris, Berlin, London - and in the other art capitals of the world.

The other major Abstract Expressionists may have made greater contributions toward establishing that movement's original identity, but it was de Kooning who pushed it inexorably toward its richest harvestings. If his ca. 1949 oil ''Gansevoort Street'' is among the most beautiful of all Abstract Expressionist paintings, then his 1950 ''Excavation'' is among the most important, and his 1952-53 ''Woman and Bicycle'' is among the most daring. But that wasn't all, for he then moved on toward a level of painterly quality seldom reached in the past half-century, producing such modernist masterpieces as ''Composition'' (1955), ''Parc Rosenberg'' (1957), ''Door to the River'' (1960), and ''Pastorale'' (1963 ).

After that, however, things began to change. His paintings became more open and expansive, more sensuous and relaxed. The extraordinary tautness and tension that had always characterized his work began to diminish, and by 1970 they were well on the way to disappearing altogether.

His new use of color didn't help, for it frequently verged on the blatant, or then again, occasionally resembled different varieties of ice cream. And yet, here and there he did manage to make it all work, did manage to come up with a painting that sang with much of the passion and authority of his earlier work at its best.

''Woman, Sag Harbor,'' ''Woman on the Dune,'' and ''Red Man With Mustache,'' for instance, are very effective, even quite exciting. And a bit later, ''Untitled IX, 1975,'' ''Untitled XIX, 1977,'' and ''Untitled VI, 1981'' also hold their own very nicely. But that, unfortunately, is about it. Very little else of his recent work should have been included in this exhibition. It's not that the other paintings are bad, only that they fall so far short of his major work as to be embarrassing.

Even so, I cannot recommend this show highly enough. It includes roughly two dozen of the best paintings produced since World War II, as many excellent drawings, and a handful of interesting sculptures. If that isn't worth a trip to the Whitney Museum, I don't know what is.

''Willem de Kooning: Paintings and Sculpture'' will run through Feb. 26; ''The Drawings of Willem de Kooning,'' through Feb. 19.