It was the 20th anniversary celebration of the opening of the nonpatisan Senate club called the Monocle on the Hill.Downstairs, the centerpiece on the buffet table was a miniature ice replica of the U.S. Capitol, while the upstairs attraction was sliced filet mignon and a bountiful raw bar.

A very exclusive Sunday evening party, as Washington political gatherings go, with 500 club members crowded into Connie and Helen Valano's restaurant -- give or take a VIP and not counting the hovering spirits of Connie's good friends Hubert Humphrey, Ev Dirksen and William O. Douglas to name but a few of the Monocole's high-powered clientele over the years.

"Connie and Helen are very special people and after 20 years some of the old regulars who've been here since the beginning thought a celebration was in order," explained Roy Elson, a Monocle club member since the Valanoses opened their doors in 1960. "This isn't just another Washington restaurant and watering hole, you know. It's an institution."

Bearing out this point, the list of VIP guests for the 6-to-9-p.m. affair was led by Vice President Walter Mondale who, while unable to attend because of delayed plane connections, did call from his plane. Mondale once went so far as to describe the D Street NW establishment as a place "where laws are debated, where policies are set, where the course of world history is changed."

It all began, according to Connie Valanos, back in 1959, when, as certified public accountants working for local restaurants, he and his wife tried to interest 15 of their clients in taking over and remodeling a small eatery called the Station View Spaghetti House.

"What I saw," remembers Valanos, "was an opportunity for a good smart tablecloth operation on Capitol Hill, someplace that the people working up here could get first-class food and service. But, would you believe it, none of my clients wanted any part of the operation. They didn't care for the location.Imagine, the Senate Office Building at your back door and they didn't like the neighborhood." So I finally said to myself, 'Okay, Valanos, if you're so damn sure the idea's good, do it yourself."

Wherever Connie and Helen Valanos' erstwhile accounting clients might have been Sunday evening, The Monocle's owners were in their element -- at the door of their establishment, greeting visitors from Sen. S. I. Hayakawa (R-Calif.) to Mayor Marion Barry, and surveying a packed house of eating, drinking Washington policians and lobbyists.

"I owned four other restaurants I didn't enjoy until we opened this place," said Connie, now 61 years old. "But this, to me, is a unique restaurant I mean, where else are you going to find the vice president, the minority leader, three Supreme Court justices, two law school deans, Edward Bennett Williams and F. Lee Bailey in back-to-back wall booths during a lunch hour? Our general manager and maitre'd, Nick Salimos, he knows how to, if you know what I mean, dress a room."

Valanos also remembers some of The Monocle's choice club members since departed -- Sen. Everett Dirksen, "who was so punctual that he'd always arrive ahead of his dinner partners and have to wait for them at the bar."; and Sen. Hubert Humphrey, for whom The Monocle's owner held a special affection.

"I'm a Democrat and a card-carrying member of the President's Club," says Valanos, "but that doesn't affect the way we deal with our customers. There is no politicking in The Monocle. This is strictly a bipartisan establishment."

Also, as The Monocle's owner relates the secret of his 20-year success, he runs a strictly Washington-oriented establishment.

"Don't misunderstand, we welcome the tourist trade," says Connie Valanos, "but our first concern when it comes to seating has always been to take care of our regular customers. And I don't just mean the senators, congressman and judges, but the staff people on the Hill who support us year in and year out. Just because somebody is a big wheel in New York or St. Louis doesn't cut any special ice with us."