"American Portraiture in the Grand Manner: 1720-1920," which opens here today at the National Portrait Gallery, is a show they ought to like in Ronald Reagan's White House. It is American and proud of it, it is conservative, theatrical, and wholly unembarrassed by the gleam of the expensive. Though it comes to town, as he did, from southern California, this scholarly exhibit says more about the formal style that rules Washington than it does about L.A.

Its 71 portraits are paintings full of props--velvet draperies (for grandeur), marble columns (for authority), harps (implying culture), puppies (domesticity), swords and flags and maps. The gentlemen we meet here strike the sort of poses we associate with statues; they point up at the sky or at the horizon. Some appear in uniform, others wear dark suits; all seem to have patronized the very finest tailors. The ladies hold a fan, a feather or a rose. They wear sheer silk, old pearls, old lace or gorgeous satin gowns whose highlights J.S. Copley or John Singer Sargent could summon into brightness with one flick of the brush.

The painters who produced these suave and sumptuous portraits--John Smibert and the Peales, John Trumbull, Gilbert Stuart, Thomas Sully, Thomas Eakins, William Merritt Chase--charged a pretty penny. A still life for the parlor, a genre scene or landscape might be made on speculation, but these grand-manner portraits were specially commissioned. Prices rose with size, a formal full-length portrait cost more than a bust-length, and a sitter would pay more for two hands than for one.

Because these portraits cost so much, and because one commission could well lead to another, the painters pulled the stops out. Most portrait exhibitions seem a little dull, but this one is enlivened by thick brocades and sheerest lace, by shining brass and leather, and other luscious passages of bravura painting. A number of these pictures--Copley's beautifully composed "Sir William Pepperrell and His Family" (1778); Gilbert Stuart's "The Skater (Portrait of William Grant)" (1782) from the National Gallery, and his famous "Landsdowne" portrait of George Washington (1796); Samuel F.B. Morse's "Benjamin Silliman" (1825); Sargent's stately, satin-gowned "Mrs. Henry White" (1883) from the Corcoran's collection, and Eakins' moving "Portrait of Professor William D. Marks" (1886)--are, by any measure, splendid works of art.

Among the many worthies here are matriarchs in ermine, soldiers wearing swords, captains (whose spyglasses suggest victories at sea), presidents and jurists, scholars and divines. Some of these were famous (George Washington, Teddy Roosevelt, John Marshall, Marshal Foch), others merely rich.

Washingtonians will detect in this exhibit something quite familiar. Its costly, often pompous, slightly Anglophilic portraits celebrate the fashion the rulers of this city have dutifully affected for more than 200 years. Politicians love the flag-and-bunting bit, the big chair at the table, the globe, the wall of books, the view into the distance. Politicians' glossies, their press-conference settings, the decor of their offices and of their Georgetown houses, borrow the pretentions, the trappings of tradition, that puff up this show.

This portrait exhibition works well in various ways, as a gathering of objects, a lesson in art history, and as a revelation of something close to home--a somber, far-from-humble sort of Washingtonian flash. It's just the sort of thing they'd study in L.A.

The Los Angeles County Museum of Art put this show together. It opened there last fall. Though organized by Michael Quick, a California curator, it is in many ways, as one might have guessed, a Washingtonian show. Without the contributions of three other art historians--all former Washingtonians and all of them much missed here--it might not have come off.

One is Marvin Sadik, former director of the National Portrait Gallery, whom Quick describes as "one of the most able and dedicated museum directors of our time . . . No exhibition of this nature could have occurred without the leadership he has provided for the study and appreciation of American portraiture." The second is Earl A. (Rusty) Powell III, who learned his business here at the National Gallery and is now director of the Los Angeles museum. The third is William H. Gerdts, who once was a professor at the University of Maryland and helped conceive the show.

Gerdts' essay in the catalog is erudite, informative. He understands, for instance, why so many chairs appear in official full-length portraits. (The throne-like chair, he writes, "is a working fixture; it explains where the subject performs his tasks and suggests a modicum of comfort--it relieves the spectator's unconscious anxiety of the strain of eternal standing." Of course it also carves the picture's painted space.) He is also very good at discussions of small details. In comparing, for example, two portraits by Thomas Hicks, an 1852 portrait of Hamilton Fish and one of George T. Trimble completed two years later, Gerdts notes that both pictures contain a wastepaper basket, "perhaps, strangely, the same wastepaper basket. It has been pointed out," he adds, "that the Fish portrait contains the first known representation of such a container--an object not then one of common domestic use--and the Trimble probably the second." The catalog is a treat. It takes us on a guided tour through this exhibition, and through a telling region of America's art history. The show closes June 6.