"For the first time in the hsitory of P Street, the White House has come to us," remarked Ramon Osuna last night as he gazed at the jostling mass blocking the view of the art mounted for the occasion. By the "White house," he meant Rosalynn Carter, who breezed in for the opening of his Osuna Gallery's five-week show of transparent cast resin (plastic casts, to the layman) by Emilie ('Muska') Benes. Benes also happens to be Mrs. Zbigniew Brzezinski.

Later in the evening, Osuna grew more circumspect, noting, "No crowd ever surprises me. I've been doing this for 20 years. Every exhibition has its own public-you never can tell who will come."

While he was making this particular observation, there was a stir at the entranceway and Osuna strained to see who had turned up this time. "Who is it?" he whispered in the general direction of the five or so guests crushed in front of him. "Averell Harriman," was the consensus.

(Harriman and his wife, Pamela, are enthusiastic owners of one of Benes' smaller pieces. "We've had one for years, I think they're wonderful," was Mrs. Harriman's response to last night's show, the first major event for the artist, who has been working this particular medium for the last five years.)

Before the Harriman's late arrival the number of powerful names per square foot had just about reach the saturation point. And by the time Sen. Jacob Javits (R-N.Y.) strolled in to give the artist a big hug and wander around contemplating her work, which was new to him ("I must prowl; I want to know if I can afford it."), it was reaching critical mass.

Mr. and Mrs. Livingston Biddle were there to herald a new art form. The chairman of the National Endowment for the Arts noted that he had never seen this particular kind of work before and proclaimed it "very interesting." Rep. Jonathan Bingham (D-N.Y.) and Rep. John Brademas (D-Ind.) , both strong congressional supporters of the arts, were stroking and discussing the clear, massive casts of tree trunks, smooth and glistening on three of the squared-off sides, rough and natural on the bark-cast fourth side.

Sen. Richard Stone (D-Fla.) was discovering the tree-trunk casts for the first time as well. "I had heard about these trees . . . and then Zbig wrote a little note . . . I was glad to have it."

The Washington art world did not go unrepresented either. Manon Cleary, Rebecca Davenport and Jim Sundquist came to honor their colleague's Washington debut, as did representatives from some other vastly different Washington worlds, James Reston, Ina Ginsberg and Deena Clark.

Still, some people will persist in asking why not the best?

"Mom is the president coming?" queried 14-year-old Mark Brzezinski.

"No, no, Rosalynn was here."

"But Dad's telling everyone he's coming."

"You'd better go talk to your father.

The president is too busy to come. It was so lovely of Rosalynn to do to."

If the national security adviser was a bit overzealous in his hopes for the evening, he could be forgiven for supporting his wife's venture. And he modestly took little credit for that support.

When asked if he had helped install the heavy pieces, Brzezinski whose smile was so wide all evening that one feared for the condition of his jaw, replied, "No, I carried them.'

"You have to make a choice," said Benes. "Either you will or won't be an artist. It's a full-time occupation." And one she decided on 15 years ago when she started working in wood and bronze.

She works, she says, "very simply" in her cellar and goes directly to nature, seeking out the trees she wants and making the rubber molds, which she then uses to form the final casts, slowly layering and tinting the resin.

"It's hard work; she's a pioneer in a new field," said Anne Kinney, who with her husband Gilbert, gave the post-opening party in honor of the artist. The Kinneys and the Brzezinskis met 10 years ago in Maine and have been summer neighbors and friends since. They were also among the first Benes collectors.

"I bought one of her pieces when she first started doing this. I've lived with it four or five years. It has a lot of character; it's very serious."

So is the artist, but the humor of the human bursts forth, too. A young art groupie poses his profound question:

"Would you say this is the typical expression of a person from your country?

"No way," hooted "Muska," eyes crinkilin to match her husband's grin, "I made it here. It is characteristic of an American living in this country."