The National Museum of American Art has acquired a major mural painting by the late Thomas Hart Benton, the 20th-century artist whose name became synonymous in the 1930s with American subjects treated grandly.

The painting, an allegorical treatment of a favorite Benton theme -- the abundance of the midwestern earth -- is to be unveiled this morning, the 96th anniversary of Benton's birth, high on a wall just inside the museum's entrance at Eighth and G streets NW.

It was partially donated by the Allied Stores Corp. and partially paid for by the museum using a portion of the Smithsonian Institution's Major Acquisitions Fund. The museum's share of the cost was $375,000.

"This is an important piece by an artist whose role in American painting is increasingly respected and discussed," commented director Charles C. Eldredge, "and it will help give a focal point for the art of the 1930s and 1940s in which the museum otherwise is rich."

Titled "Achelous and Hercules," the painting focuses on the combat between Achelous, the river god in the form of a rampaging bull, and the Greek hero in the guise of a sinewy American farmhand. In the mythical story the god is no match for the hero, who snaps off one of his horns. Similarly, in Benton's version the detached horn is transformed miraculously into a cornucopia symbolizing the taming of the raging river and the consequent bounty of the land.

The painting, measuring 5 feet high by nearly 22 feet long, achieves Benton's avowed aim of creating a "compact, massive and rhythmical composition of forms . . ." From left to right, the elongated figures in the painting seem engaged in a dance that, for all of its drama, unfolds at a rapturously slow pace. The viewer's eye is free to follow this action along the sinuous line of arms and bodies, plants and hills, that the artist provides, lingering along the way upon plentiful details -- the ax relaxed against a cut log, the female hand extended in a crowning gesture, the meticulously rendered fruits of harvest.

It is at once a bit cornball and a bit majestic -- an effect Benton may have intended when he wrote that regional art should symbolize "what the majority of Americans had in mind about art -- America itself." The central grouping and farmhand stripped to the waist are especially poignant. The young man seems dangerously close to losing his balance, and yet his right hand is poised at the point of the bull's horn exactly as Michelangelo, in the Sistine Chapel, poised the hands of God and man. In the background a cinematic cowboy raises his hat in encouragement and tribute.

Benton, born in Missouri in 1889, was among a legion of American artists of his generation who went to Paris and became enchanted with the new painting there, but in the 1920s he turned his back emphatically upon the avant-garde to champion home-grown subjects. He became the best known and most vociferous of the American Regionalist painters, and throughout the 1930s and 1940s he was busy painting murals for public and private clients.

"Achelous and Hercules" was painted in 1947 on commission by Harzfelds department store in Kansas City, Mo., where it was installed until last year, when the store closed. Harzfelds was purchased by Allied Stores Corp. in 1981. The Smithsonian's Major Acquisitions Fund was established in 1977 with money from the institution's profit-making enterprises such as the Smithsonian Magazine and museum shops.