ONCE, AT a party at the vice president's house, a man came up to Walter Mondale and pointed to a painting on the wall. "Isn't that a Diebenkorn?" he aked. The vice president went to check, and sure enough it was.

At a show at the National Gallery of Art, the same man surprised director J. Carter Brown by recognizing a certain piece by Spanish sculptor Julio Gonzalez and asking why it was placed with the David Smith sculptures.

This connoisseur is neither academic nor museum director. He is Sidney Yates, 70, longtime Democratic congressman from the North Shore of Chicago, who once collected a bit of art himself.

"Too expensive," he says now, matter-of-factly.

He is also the chairman of the House Appropriations Subcommittee on the Department of the Interior and Related Agencies. That means that most people have never heard of him unless they work on parks or staff the National Gallery of Art, the Smithsonian or the National Endowments for the Arts and Humanities. To those people, he is the essential congressman -- the man who, for six years, has grilled them in hearings, defended their appropriations bills as they moved through committee and onto the floor of the House, and is one of the most knowledgeable, most respected and most powerful advocates for the arts in Congress.

He also takes a strong interest in local culture, especially the financially troubled National Symphony Orchestra. Washington, he believes, "is becoming one of the great culture centers of the world. We have great art centers. We should have great music centers. I can't conceive of a nation's capital without a symphony." He shrugs and asks, "But what do you do about it?"

He has done a lot. He suggested the summer NSO concerts on the west lawn of the Capitol -- for which the NSO last year received $300,000.

And last summer Yates suggested that Wolf Trap -- which received funds from the Park Service, also under Yates' jurisdiction -- restrict itself to a flat fee from the symphony's appearances and gives the remainder of the box-office take to the NSO. Wolf Trap officials were not pleased, but agreed anyway. "I don't think it worked very well for us or them," said Kay Shouse, donor to Wolf Trap. This year, Wolf Trap is back to a percentage contract with the NSO. The Symphony did in fact earn more last year at Wolf Trap than they had in the past, but still took a loss, according in NSO president Martin Feinstein.

Yates has also talked to Roger Stevens, head of the Kennedy Center, about letting the Symphony play rent-free in the Center's Concert Hall in exchange for forgiving the Kennedy Center's multi-million dollar debt to the federal government. "Maybe we can work something out," said Yates. "But I'm not going to hold it over [Stevens'] head."

Yates goes regularly to parties and openings. I like talking to Carter Brown, I like talking to Dillon Ripley [secretary of the Smithsonian]. Generally he doesn't talk business. We might talk birds."

Yates "prizes those social invitations," said one arts observer. But he maintains a low profile. He doesn't have to do otherwise: Everyone at a party will eventually make their way over to hime, to talk, to lobby discreetly. Yates will listen, talk, banter. Seeing Larry Chernikoff, NEA's congressional liaison, talking conspiratorially to NEA chairman Livington Biddle at one party, Yates called to them, "What are you talking about, Larry? Let's hear it."

"baseball," said Chernikoff with a grin.

"No wonder you don't know anything about the arts," quipped Yates.

Many people feel that Yates is more important then he's given credit for. "One can spend a lot of time authorizing legislation, but it comes down to who's fighting for the money," said Mike Dorf, Yates' special counsel. "I don't think he's gotten credit for being responsible for federal funding for the arts."

According to some observers, that bothers Yates -- to the point that he feels a certain rivalry with highly publicized supporters of the arts like Rep. John Brademas (D-Ind.).

"He sees himself confronting that Senate but then he sees Brademas getting a lot of attention for the early legislation bringing the Endownments into existence," said one observer. "That burns him."

Brademas and Yates both deny any rivalry. "I feel a partnership with Congressman Brademas," said Yates. "I don't look for credit. As long as programs do okay, that's fine. People know of my work -- people in museums know."

So do those who attend his annual appropriations hearings for the Endowments, where his dry wit sometimes ends up the star of the show. At the hearing last spring for the NEH, Yates inadvertently stole chairman Joseph Duffey's moment of drama. Duffey had planned to hold up an old book by the binding and letting it fall apart, demonstrating the need for funds for preservatmon. Yates picked up the book and read the title. "It's Captain Bligh," he said. Then he rifled through and began reading nonchalantly, "She approached him with a slight hesitation. . . ." Giggles ran through the whole hearing room. Yates read some more, shook his head, and deadpanned, "That has to be saved."

Yates' Washington office walls reflect his interest in art. On them hang an original Roy Lichtenstein slik-screen, now faded by the sunlight that streams in; Hirshhorn Museum opening posters by Willem de Kooning and Clifford Nolan; a Johnny Firedlander print. There are also photographs of Yates playing golf, a sport he pursues avidly. He is tan and looks healthly and younger than his 70 years.

He has been a congressman since 1948, living on Lake Shore Drive in a six-room apartment all that time with his wife Adeline, who now sits on the board of the National Symphony Orchestra. His son, Stephen, is an associate judge of the Cook County Circuit Court in Chicago.

Yates' district contains some of the wealthiest Chicagoans who live along the lakefront and some of the poorest who live in northwest side neighborhoods. tYates once roamed the northside galleries of Chicago, meeting up-and-coming artists like sculptor Richard Hunt, now a friend and a very successful artist. Some of them helped him in his campaigns.Others Yates helped. Architect Mies van der Rohe's daughter had a problem immigrating here in the mid-'50s. "There was no real reason for techinical objections to her coming," said Yates. "I helped her get some papers from the appropriate sources."

He has always been interested in the arts: He memorized almost all of Gilbert and Sullivan's songs when he was in college and law school at the University of Chicago. He once studied pre-Columbian sculpture in Mexico. u

In 1962, he left the House to run unsuccessfully for the Senate against Everett Dirksen. He returned to the House in 1965. "I liked the work in the Interior Committee, and I asked to go on," he says. Following the line of seniority in the subcommittee, he assumed the chairmanship in 1974.

"To my mind, Appropriations is the most interesting," says Yates, who will never say art is his favorite area. (His principal interest is "urban problems.") "Mr. Natcher has the Department of Health. We have energy, fossil fuels, alternatives to coal, and then we have the great museums -- the National Gallery, the Smithsonian, the Endowments."

Some of his admirers say wistfully that if he hadn't given up his seniority in the House, he could be chairman of the full committee by now, maybe even House majority leader or speaker.

"He's probably one of the brighest and most effective at getting legislation through the House fairly intact," said former Yates staffer Max Richtman. And Yates has made a reputation in many areas. He drafted the amendment against funding for the SST in 1971. Yates also helped solve the Food Stamp funding crisis this spring by suggesting that funds for it be removed from the supplement appropriations bill and handled separately.

"He's also sort of the unofficial dean of the Jewish congressmen," says Richtman. "They get together when they see it's necessary -- like during the war in the Mideast in '73."

"I argue [in committee] for the arts on the basis of testimony and in the national interst," Yates says. "Many people are interested in all aspects of art. This was in the mind of our founding fathers. I remember a statement of John Adams. He proposed to study the sciences and philosophy so that his children might study arts as well."

Each spring, in a packed, overheated hearing room on the Hill, the sharpeyed, frowning Yates sits before an array of eager, scrubbed-looking staff members from one of the Endowments -- each has its separate hearing -- armed with thick books of records and statistics. Yates systematically grills them on a broad range of topics -- from why an Endowment can have both a deputy chairman and an assistant chairman, to why they don't ask for profits on lucrative productions they helped finance, to whether or not a certain college was denied a grant because of elitist reviewers at the Endowment.

"It was not the easiest hearing," said Joseph Duffey, chairman of the NEH, who was asked some of those questions and more this spring. "He takes the hearing process seriously -- which some don't."

But when it's all over, Yates turns around and gets the Endowment budgets through the full committee and through the House with no problem. This year was no exception: Last month, the House approved the Interior and Related Agencies appropriatons bill which included $160.1 million for the Arts Endowment and $152.2 million for Humanities.

"Not a single thing was changed when his bill went through," said one Hill staffer. "The others drag on for days and lots of amendments and fights. His went through in one day."

"It was beautiful to watch," sighed Yates' counsel Dorf. "Every other appropriations bill so far has been hit by an across-the-board cut. Clarence Brown (R-Ohio) asked for cuts. But he didn't do that to Yates." Yates talked to Brown and showed him there was no fat in the budget, Dorf said.

Yates also does his homework on the arts. "He's the only member of the appropriations committee who comes by the Endowment regularly," said NEA chairman Biddle. "He likes to chat with the program heads."

"There's no question in my mind that anything that interests Yates makes an Endowment staffer think twice," said one NEA staff person. "Yates is a man of strong understanding and interests, but he keeps himself totally separate from the grant process. When the Symphony was cut back in funds this year, we didn't hear a peep from his office."

"That's the astounding thing about Yates," says Mary Ann Tighe, deputy chair of the Arts Endowment. "He keeps hands off."

But one local arts source said some people at the Endowment do feel pressure: "They get appeals from his office. It's never a strong-arm thing. pBut the very fact that it's mentioned makes an Endowment staffer wriggle."

"I can't imagine enjoying a Congressional hearing more than one before Yates," sayas Tighe. "It becomes a sort of annual catharsis for this Endowment. You have to sit down before you go to him and think hard about everything you've done the year before. He restores your faith in politicians."