"Our music is remembered when their politics are forgotten," says one of the lead characters in Peter Shaffer's play "Amadeus." There was no perceptible reaction to the line from an opening-night audience of primecut Washington celebrities at the National Theatre last night, but they did give the play a standing ovation at the end.

"Amadeus" is about Wolfgang A. Mozart, depicted in this more-or-less historically accurate play as a foul-mouthed, eccentric genius, and his slow destruction by a jealous rival composer, Antonio Salieri. And just as the composers of Mozart's time were dependent on the favors of Emperor Joseph II and his courtiers, so are the arts today dependent on members of the mink-wearing class such as those who were invited to last night's premier. The National Theatre is under new management, nurtured by a new nonprofit corporation and the Shubert Organization, and so a splashy opening night was called for and delivered, with hopes that some of them might contribute to the new enterprise.

"She looks good," said a young woman in the audience as Lady Bird Johnson was escorted to a seventh-row seat with her daughter Lynda Robb and the Secret Service in tow. The aisles were clogged with cheek-kissers and "darling!" criers, men in tuxedos and women in evening chic who took a long time finding their seats because of the need to stop and chat on the way.

The crowd was about evenly divided between Republicans and Democrats, said Maurice B. Tobin, who chairs the National Theatre Corporation's Board of Directors. There were few hints of the impending change in administration, as everyone was very well-behaved and not likely to either sulk or gloat.

"Well, hello there," said one man to a friend, "Are you guys ready to take control on the Hill?" Heh heh, they both laughed.

"I'm really removed from all this since I live happily in Texas," said Mrs. Johnson, who said she had not predicted President Carter's dramatic loss or the shift in the Senate from majority Democrat to Republican. She loved the play though. "I was just overwhelmed."

"Are we cheerful? How about euphoric?" said lawyer Michael Butler, a partner of Jim Baker who was reported yesterday headed to be chief of staff for Ronald Reagan. "Of course many of us are very sobered by the responsibility too," added Rep. Tom Evans (R-Del.), whom Butler described as one of the few congressmen who can have a state delegation caucus while he's shaving. (That means there is only one congressman from Delaware.)

About 400 of the audience were invited to an after-theater supper at the Corcoran Gallery of Art, where Peter Duchin played dance music and waiters served Pasta Amadeus and pate au poivre. Round tables were set up for the diners, decorated with wild flower centerpieces.

"I was amused at the theme of the play, which is pegged to the idea of mediocrity," said Sen. Richard Stone (D-Fla.). "It's typecasting for the audience." Sen. Stone will not be a Sen. much longer, as he was defeated in a primary contest. And no, he said a tad gruffly, he'd rather not say what he's going to be doing next.

"I feel so badly about Richardson," said Rep. Lindy Boggs (D-La.) after greeting Rep. Richardson Preyer (D-N.C.), another casualty.

"This [opening] is a culmination of five years of effort," Tobin said. "We promised the best plays, the best actors, the best theater. If you have good theater, I don't care if it's in a pup tent, people will come." That may be, but mink will never do in a tent.