SIX PAINTERS and one sculptor - all American art heroes - have been selected for inclusion in "the core sample of the abstract expressionist movement" now on exhibition at the National Gallery of Art. Their show is called "American Art at Mid-Century: The Subjects of the Artist," and its timing is just right.

Arshile Gorky, Jackson Pollock, Willem de Kooning, Mark Rothko, Robert Motherwell and the sculptor David Smith began fighting for abstraction just after World War II. Together they were scorned, together they earned fame. Everyone who studies this country's modern art knows their famous images and their often-painful stories. All of them have been devotedly promoted. Myth surrounds them still.

Two of them - Rothko and Gorky - were suicides. Smith and Pollock died early, in road accidents. Barnett Newman painted his "Stations of the Cross" between his heart attack and his death. This set of austere canvases is subtitled, in Hebrew, Lema Sabachthani, "Why hast Thou forsaken me?" Often in this show that sort of cry is heard. De Kooning spent more than two years slashing at the paint of his "Woman I." Equally obsessively, Motherwell has painted a single heavy image, of ovals and black bars, more than 140 times.

They no longer seem a team. In the seven one-man shows that form this exhibition each artist stands alone as if waiting to be judged. Art history, we know, frequently demotes the giants of the last generation. How good were they really? Are these seven men worthy of the praise lavished on their art by their champions? Will their work be seen in the National Gallery of Art half a century from now? It has been 30 years since these seven heroes entered the arena. Now posterity must judge them: Thumbs up or thumbs down?

Only David Smith and Jackson Pollock, it seems to me, are unquestionably masters. Together they rule this show.

Beside Smith's Voltri sculptures and the "classic" 1950 poured paintings of Pollock, Gorky, De Kooning, Newman and Mark Rothko seem overshadowed. All six, however, tower above Motherwell. A painter at once blunt and chic, he is not in their league.

Such judgments may be skewed by the present exhibition. If any suite of paintings made by an American in the post-war years can stand be competition of those Pollocks and those Smiths, it is the cycle of dark pictures that Mark Rothko painted for his Houston chapel. But Rothko here is represented by a group of lesser works, on paper, done in 1969 just before he died.

Curator E.A. Carmean Jr., who chose this exhibition, has been fairer to Gorky, Newman, and De Kooning. To see De Kooning's "Woman," and the pictures Gorky called "The Piow and the Song," and Barnett Newman's "Stations," is to see these painters at their finest. But here, beside the Smiths and Pollocks, Gorky's finest efforts, as those of Newman and De Kooning, seem slightly lesser works of art.

Gorky above Motherwell, Smith above De Kooning - it may, of course, be said that such rankings are unjust, premature, subjective. A dislike of the minimal may diminish one's approval of the works of Barnett Newman just as a bias against expressionism's messiness may blind one to the beauties of the "Women" of De Kooning. Also one may argue that there is no competition, that these seven one-manexhibitions must each be viewed alone. But there is something in this show, or shows, that warrents cross-comparisons. Smith is an exception, but there is much the others share.

They each painted in series. Each of them selected what Carmean terms "a subject," but what might be better called a kind of trademark image. We know Newman by his "zips" and stripes. De Kooning by the slashings of his loaded brush, Pollock by the tracery of his woven fields. Motherwell has painted his black, testicular device time and time again. These painters, unlike Rembrandt, say, or Picasso, or Matisse, did not move from portraits to histories or landscapes. Instead they closed their worlds. Perhaps because they had dared to test the void of abstraction, they felt a need to place limits on the universe, and then work within them. There are 15 Newmans here, and 10 "Elegies" by Motherwell. In a sense their paintings are not many, but one.

Though the greatest masters of the past, the Shakespeare and the Rembrandts, seemed to know to boundaries, one can make fine art within limits. Vuillard found the world in his small French apartment; Monet spent his last years painting mighty masterworks within the confines of his garden; Matisse, with nothing more than shears and colored paper, could conjure goddesses and seas; and Mondrian discovered all that he required in rectangles and lines. One must judge the painters here not by the boundaries they set, but by what they found within them. What moves us in the works of Smith and Pollock is the largeness of their souls.

Though De Kooning spent two years on a single picture, Smith, amazingly, made his 26 Voltri sculptures in just 30 days. Some of them are huge. All are made of scraps of heavy steel - sheets, old tongs, anvils, and old wheels - that the artist found in five abandoned factories in the Italian town of Voltri in 1962. Yet he somehow managed to produce works that seem both wise and witty, new as well as ancient. Like the cut-outs of Matisse, which both predict the future and summarize the past, the Voltri sculptures - which have been beautifully installed here - seem not minimal, but maximal. They somehow seem to call up heroes, seas, suns, dancers, still lifes and the chariots of the gods.

The Pollocks, too, seem endless. To stand before "Lavender Mist," as it breathes in changing daylight, is to see a kind of glory, measureless and bright.

Something just as vast is found within the near-black Rothkos of the Rothko Chapel. But while Pollock's fields show us energy that dances, Rothko, instead, shows us something close to death. Shadows, ghosts, and darknesses drift and disappear in Rothko's huge and looming voids. His Brown and Gray paintings, now hanging at the Gallery, seem, in contrast, only sketches. They are not unhandsome, but they seem to be first steps towards pictures never made. They also yearne for daylight. Seen beneath electric light, these horizon-broken fields seem a set of frozen seascapes not yet brought to life.

Both Gorky, in the 40s, and De Kooning, in the early 50s, seemed valiant and adventurous explorers of the new. But while the Pollocks, painted circa 1950, have in no way dimmed or dated, those of Gorky and De Kooning seem tethered to the past. Gorky called his paintings "windows viewing infinity," but they do not look out as much as they look in.

Gorky was Armenia born, and nostalgia for his homeland fuels his lovely paintings. In his evocative and flowing biomorphic shapes Gorky could discern apricots and wheatfields, "the beautiful Armenian slippers father and I used to wear," and "Mother's soft Armenian butterchurn," but his reveries are private. The series on display is called "The Plow and the Song." ("What I miss most are the songs in the fields. No one sings them any more," the painter wrote. "And there are no more plows. I love a plow more than anything else on a farm.") His works are somehow veiled. Before these delicate pictures, despite the sweetness of their music, the viewer feels left out.

Art teachers used to link Pollock and De Kooning as if the two were twins, but with every passing day the gap between them widens. While Pollock seems a prophet, De Kooning seems, in contrast, a kind of latter day practioner of the Dutch tradition. There is something of Franz Hals in the way he moves his paint, and a bit of Rembrandt in the scale of his figures, and there seems to be a touch of Northern European violence in his often savage art.

Those who heard him speak remember Barnett Newman as a kind of metaphysical Old Testament New Yorker. He had wisdom, courage, passion, and he gave all that he possessed to his "Stations of the Cross." He saw them as religious pictures, passionate yet still, tragic yet affirmative, but I am far from sure that viewers of the future will take out of these paintings all that he poured in. Seeing God, or life and death, in this processional composition requires a leap of faith. I find them deeply moving, but skeptics may regard them as just black lines on white.

Of the objects in this show, the Gallery owns three - a subtle Gorky drawing, Pollock's "Lavender Mist," and Smith's terrific "Voltri III." Carter Brown, Carmean, and the Gallery's trustees chose these objects will.

When they began their battle, in the 40s and the 50s, many of these artists felt embattled and alone. They did not prosper early; they were scorned and often laughed at. They changed the history of painting, bu their struggle for abstraction long ago was won. Their sources and chronologies, debts and innovations, no longer seem important - except to art historians.(A most painstakingly researched 268-page catalog, by Carmean and Eliza E. Rathbone, accompanies this show.)

The only thing that matters now is the beauty, and the power, and the presence of their art. The Pollocks and the David Smiths seem certain to survive. Rothko's Houston Chapel will be a goal of pilgrims for many years to come. Gorky was a poet, De Kooning wields the brush with a master's touch, Newman had great courage. But posterity is ruthless. For each good painter who achieves what in the world of art is Olympian immortality, there are tens of thousands who, through no fault of their own, are excluded and forgotten. All these men are worthy, and all of them fought valiantly, but in the history of art, as in the Roman games, even valiant fighters die.

This show is not without its flaws. The Rothkos and the Newmans seem installed too close together. One misses certain painters, especially Clyfford Still, and also Adolph Gottlieb. But to see that room of Pollocks, that gallery of Smiths, and the other pictures here, and to see them at this time, is both a privilege and a challenge. No tickets are required for admission to "American Art at Mid-Century: The Subjects of the Artist," which will remain on view through Jan. 14, 1979.