THE ART world here is at a loss. Walter Hopps is leaving. There is no one to replacel him. No curator in sight can match his underground connections, his eerie potent power, his willingness to look, his exhibition record, his elusiveness, his fame. Now Hopps is off to Houston. His presence here is fading. He is going . . . going . . . going . . .

But he is not yet gone.

His legacy surrounds us. His influence still holds. In 15 years in Washington, sometimes in the foreground, directing art museums, often in the background, pulling hidden strings, he changed the way we see.

Even now as he commutes to his new job in Texas, Hopps is still promoting artists he encountered here. His name is still appearing in unexpected places. He edited the photographs in the drugs-and-decadence biography of Edie Sedgwick that is now on the best-seller lists. And he's still arranging shows. One, a small exhibit of sculptures made in Washington and in California, is now on view in Oakland. A second, a traveling exhibit of assemblages and photographs by Washington's William Christenberry, will be seen next month in Houston. A third, "Poetic Objects," -- "it may well be," he says, "my last from-scratch exhibit organized in Washington" -- will open here next season at the Washington Project for the Arts, where Hopps is on the board.

"In all the years I've been here, I've been longing for the West," he says. "I still live in Washington, in Washington and planes. I travel all the time -- to Houston, New York and L.A. Within a couple of years, I'll be living in Houston. And traveling to Washington. You'll still see me around."

He is lean and tall and dark. Even now, at 50, he often seems to speak with an adolescent's urgency. He chain smokes and he fidgets. His friends have often noted the extraordinary energy, it sometimes feels like craziness, that seems to crackle in him. He manages to seem at once both confident and anxious, pedagogical and secretive. His gaze is so intent, he is so sharply focused, that he seems to communicate more than his words contain.

He is speaking now of Houston and of his newest patron, Dominique de Menil, and of her links to oil, to France, to the church and to Northern California. He patiently describes the works of art she owns, the Max Ernsts, the Magrittes, the Mondrians, the Rothkos, the figurines from Mali, the works of Celtic iron. As he speaks he seems to see them in his mind in the museum she is building, the new Menil Collection, the $20 million plant that Hopps was picked to run.

"It won't be like the Centre Pompidou in Paris. You want to cry when you see a Vuillard in that spaceship machine. It won't be like the East Building, transportation seems the core icon of that building -- the visitor is processed commodiously and swiftly as he might be through the finest airport in the world. Its iconography will be instead part workshop and part house. Its colors will be gray and white. The daylight in the galleries will be reflected twice from polished concrete shells before it hits the walls. It is going to be a beauty. It will have the lines of a fine yacht."

Without cutting ties, Hopps is starting over. He has made such moves before.

When he arrived in Washington, in 1967, from his native California, The New York Times described him as "the most gifted museum man on the West Coast (and, in the field of contemporary art, possibly in the nation)." Hopps justified that billing here. How he did so is not clear.

In everything he did he counfounded expectations. He wrapped himself in mystery the moment he appeared. Everyone who met him recognized his gifts -- his talk was incandescent, his knowledge overwhelming -- yet no one could predict what art he would show next -- '50s New York photographs, the laser beams of Rockne Krebs, the paintings of Frank Stella or the comics of R. Crumb. His suddenness and patience were equally astounding. He seemed to be committed to complicated strategies, trivial or global, private or eccentric, that no one else could follow. What was it that moved him? What voices had he heard? No one could be sure.

"He's the most mercurial person I've been close to in this business," says artist Ed McGowin. "He was always inexplicable -- sometimes to a fault, more frequently in ways that turned out to be magical. I never understood him.

"Anyone who says [he] understands him lies." HOPPS' career in Washington has been studded with surprises, gradual dissolves, disappearances, defeats. The Washington Gallery of Modern Art, the first museum he directed here, did not survive his reign. It merged into the Corcoran and exists no longer. As director of the Corcoran in the early 1970s, Hopps proved unable to put right the mess he found there. He struggled for a while, then took a leave of absence in 1972 and did not return. His tenure as the curator in charge of contemporary art at the National Collection of Fine Arts also ended with a fade-out in 1980. This is not a string of triumphs. But as often as he's vanished he has reappeared.

He is as busy now as ever. Among the shows he is designing, one is bigger than the others. It is not aimed at Texas. A kind of exposition tentatively titled "The Automobile and Culture," but known better as "the car show," it will open in Los Angeles 10 days before the start of the 1984 Olympic Games. It has a $1 million budget. Hopps has been invited to organize the car show by the new, not-yet-open Los Angeles Museum of Contemporary Art.

Pontus Hulten, the Swede chosen by the French to be the first director of the Centre Pompidou in Paris, is the first director, too, of the Museum of Contemporary Art.

"I've known Walter many years," said Hulten. "In the early '60s we pursued parallel ideas. He was interested in Rauschenberg and Duchamp. So was I. When I first came to this country, I called him up -- from Tokyo -- and told him I was on my way to California. He met me at the airport. We got into the car and he took me to Watts Towers. He thinks the way an artist thinks. He has an artist's passion. He has never cut his ties to Southern California. First as a dealer, and then as a promoter, he helped create this city's art. He seemed to be behind everything that's good here. And so we turned to Walter. It was an obvious obligation." "I KNOW no other curator who works as well with artists," says sculptor Rockne Krebs. "We are no longer close, we don't see each other much, but once he changed my life. He was remarkably encouraging. He'd take the time to talk, to teach you the art world's ways. He'd talk to other curators, and he had such credibility, they would fly in from Chicago just to see your art. His timing is unmatched. Think of what he did for Robert Rauschenberg. His show made Rauschenberg the American artist during the Bicentennial year. He is going to make a hell of a lot of difference in Bill Christenberry's life. Hopps is going to move Christenberry into national prominence.

"Walter is an artist. He doesn't make art. He is an artist who makes shows." MOST OF the directors of Washington's museums are solid, desk-bound sorts. But Hopps was not like that. If you ever tried to call him, at the Washington Gallery of Modern Art, or later at the Corcoran, you know how hard he was to find. Was he holing up again, with a pile of his papers, in a Washington hotel room, or visiting New York, or prowling studios in Chicago? His phone would ring and ring.

"He had promised me a show in 1969," said photographer John Gossage, "but nothing had been settled. We had a hundred things to talk about, but I could never find him. I would call and call. Three weeks before the opening -- I had almost given up -- I stepped into a subway car at 96th and Lexington and there was Walter. Without breaking stride, he answered all my questions and told me what he wanted. By the time we got downtown, everything was set."

There are California artists, old friends of Walter Hopps, who believed that he had never left Los Angeles. He would drop in at their studios. They would see him at the bars. When accused, not long ago, of abandoning this city for his new job in Texas, Hopps denied the charge. "I never left Pasadena," he said.

"I saw Walter a few days ago," said dealer Ramon Osuna. "I said, 'Where are you living nowadays?' 'Everywhere,' he answered."

Hopps' true home seems to be not one place, but many. He is as well known in Chicago as he is in Dallas. He has helped mount exhibitions at the Philadelphia Museum of Art, at New York's Metropolitan, in Venice and Sa o Paulo. Hopps appears to belong to a vast extended network, a kind of art-world web.

That thought-field, that web, wrapped every exhibition that he mounted here. Hopps' mind seethes with memories and histories, cross-references and gossip. His shows were just as full. Each object he displayed -- that Diane Arbus photograph, that portrait by Joe Shannon -- seemed linked to the others. Most curators of modern art specialize in this or that. Hopps proved himself a pluralist long before that term was in fashion. His exhibitions here -- the group shows he arranged that were open to all comers, his one-man retrospectives -- eventually acknowledged the whole writhing, always shifting world of contemporary art. WHEN HOPPS was at the Corcoran, Hal Glicksman, his assistant there, had metal buttons printed. "Walter Hopps," they read, "will be here in 20 minutes." Everybody laughed. Hopps was often late.

More often he's been early.

Hopps may be the sharpest art scout of his generation. Stretched behind him like a tail is an honor roll of artists, a record of exhibits, of discoveries, of prophecies, that no one else can match.

What we know today of Marcel Duchamp's secrets, of Joseph Cornell's boxes, of Robert Rauschenberg's coherencies, we owe in part to Hopps. It was Hopps who organized their first museum retrospectives.

Hopps was only 21 -- his friends still called him "Chico" -- when, in 1958, he and sculptor Ed Kienholz opened a small gallery, the Ferus, in Los Angeles. Unlike many of his colleagues, Hopps has had, since high school, a particular affection for the secrets and mind games of the Dada and Surrealism. But his mind was not closed. The Ferus early on showed Billy Al Bengston, Larry Bell, Robert Irwin, Ed Ruscha and Paul Sarkisian. And it was the Ferus Gallery that gave Andy Warhol his first one-man show.

In 1962, Hopps signed on as curator with the Pasadena Art Museum. There, before the term was coined, Hopps displayed the Pop Art of Wayne Thiebaud, Jim Dine and Roy Lichtenstein. Before a year had passed Hopps was directing the museum.

For a quarter-century his eye has been a kind of moving spotlight, illuminating quality. The artists on his honor roll, whose works he has exhibited in shows both large and small, include New York's Barnett Newman and Chicago's Hairy Who, the photographer Diane Arbus, the sculptor H.C. Westerman, the color painter Howard Mehring, Larry Poons, Sam Gilliam, Kurt Schwitters, Gene Davis, Richard Estes, Jonathan Meader, Joe Shannon, Philip Pearlstein, Lou Stovall, Yves Klein, Louis Faurer, S. Clay Wilson, Yuri Schwebler, George McNeil, Anne Truitt, Gage Taylor, Robert Gordy, William Eggleston . . . The list goes on and on.

Zap Comix and photographs, assemblages and carvings and color field paintings -- it is the strangeness of that mix that makes his judgment special. No theory rules his taste.

"You can't say how he picks them. His nerve endings just tingle," says photographer John Gossage. "He could look at a brick wall, and if one brick there had genius, he'd pick it in a flash."

"He is absolutely brilliant, true, but difficult to work with," says Richard Murray, who helped Hopps arrange the Rauschenberg retrospective at the National Collection of Fine Arts. Murray now directs the Birmingham Museum of Art. "He would search himself so carefully he sometimes seemed to dawdle. But once he made a decison, it always seemed the right one. His eye is unerring. His knowledge is intricate and overwhelming. He knows things about art, about its byways and its back paths that no one has considered. He couldn't meld all his ideas with an institution. That does not mean he failed."

Some people have a way with cats or dogs or horses. Hopps has a way with artists, but not with artists only. Many are the dealers, the critics and collectors, who have also felt him blaze.

"Hopps is a connector," says dealer Harry Lunn. "He negotiates, conciliates, advises and befriends, but does so from the background. He's been a kind of eminence grise here, the Habib of the art world. He used to be a dealer, so, of course, he understands us. He is as skillful with collectors. What I find most amazing is his power of prediction. Look what he's done for Bill Christenberry. In a sense, Walter has been working on Christenberry's Houston show for the past decade. Long ago he saw virtues in the work that were not yet there, it was as if he'd glimpsed objects that the artist had not yet created." IF HISTORY forgets him, blame it on his writing, or rather, on his lack of writing. Though articulate, Hopps seems to dread the act of putting words on paper. "We're all still waiting for our catalogues," says artist Ed McGowin. "Walter ought to have a scribe," says Krebs. "It may be," suggested Anne d'Harnoncourt, director of the Philadelphia Museum of Art, "that Walter sees cross-references so clearly, and sees in his mind's eye so many different objects, and has so many things running through his mind, that he can't put one thing down."

Hopps' accomplishments in Washington cannot be found in libraries. The vision that he showed us exists in our memories, in his viewers' minds.