Reprinted from yesterday's late editions.

Betty Ford did it for the dance; Lady Bird Johnson did it for spring flowers. Wednesday night - for the first time since her husband, Walter, was elected Vice President - Joan Mondale lent her new prestige to the cause that she holds dear.

Before 125 museum professionals gathered for a dinner at the National Gallery of Art Joan Mondale spoke about museums.

Joan Mondale, as the art world knows, is a museum professional. She has worked for the Boston Museum of Fine Arts, for the Minneapolis Institute of Arts and for the National Gallery of Art. Mrs. Mondale is a student of this country's art and artists. Five years before the pictures of artist Robert Rauschenberg made the cover of Time magazine, one made the cover of her book, "Politics in Art." She knows whereof she spoke.

"We have never close to the top who really knew, and really cared, about art museums," said Washington's Roy Slade, director of the Corcoran Gallery of Art.

"Last week," said Slade, "Mrs. Mondale spent two hours at the Corcoran Biennial. She wasn't looking for publicity, she was looking at the paintings. Since I've been at the Corcoran, the first and second families have pretty much ignored us. We invited Nelson Rockefeller time and time again, but he never came. When he took his office we hoped that he'd become a champion of the arts. The only thing he did was buy that Max Ernst bed."

Every museum," said Joan Mondale, "teaches the lesson that there is no substitute of the real thing . . . In a world where so much is artificial, museums have the authority of the geniune."

"Museums," she said, "are a major and irreplaceable national resource. They plays a role that no other institution in our society can play. They enrich the mind and challenge the imagination. They conserve knowledge and they give pleasure.

"It is important," she told her audience of trustees, "not only that you take pride in your achievement, but that you be recognized for it by your nation and your nation's government.

"Museums," she said, "lay before us all the things that people take or have taken seriously . . . They are complete where your and my education may have been spotty. They help us fill the gaps . . . Most of all museums let us draw our own conclusions. Any exhibit may have a point behind it, but we have to arrive there on our own. And once we get there, the point sticks.

"Gertrude Stein remarked of the Museum of Modern Art that it could be modern or a museum but not both. I think Miss Stein had it wrong. For the artist, all art is contemporary.

"Museums," she continued, "have been changing . . . Old definitions are ceasing to apply. Boundary lines are breaking down. Art forms are no longer distinct and separate from one another.

"A museum breaks down boundaries. It educates. It maintains standards. And it must be open to all. It is," she quoted, "this place where the best of man has been put."

"What, then," she asked, "is a museum? 'What is fat, what is thin, large, small, good, bad, young, old, has many heads, eats money, is fuzzy, gives itself with joyous abandon, and is loved by everyone?'

"It is my privilege," she told her audience of museum professionals, "to tell you how much you are loved."

She addressed the Trustees Committee Spring Conference of the American Association of Museums at a dinner given in their honor at the National Gallery of Art.