WHEN THE East Building of the National Gallery of Art was in the planning stages, architect I.M. Pei and director J. Carter Brown moved miniature sculptures around a mock museum to see how they worked in the space. Those sculptures were baby models of David Smith's life-sized work.

Standing in a gallery of Smith's steel and bronzes last night, Brown said with obvious pleasure, "Now some of those same sculptures are here." Of the tricky placement of Smith's large-scale works, Brown added, "It was a gamble because it took us five years to get up the courage to do this. The building itself is so strong artistically."

The black-tie opening, and the perusing at the 62 sculptures weighing a collective 35 tons, was dominated by art patrons, philanthropists and business associates of the American Medical Association, the underwriter of the show.

Among the guests from across the world, such as Texans Joan Herring and Ruth Carter Johnson, and Julian and JoAnn Ganz of Los Angeles, were a few familiar Washington faces such as Deputy White House Chief of Staff Michael Deaver.

He characterized the post-election mood at the White House "as very high, very happy," and explained, "This is the first time since 1928 that a Republican president has maintained his Senate majority in a mid-term election." On the anticipated new spirit of compromise between the White House and Capitol Hill, Deaver said, "It's too early to say -- depends on the issue."

Sen. Bill Bradley (D-N.J.) called Tuesday's results "a victory for balance and moderation. The Senate will be a different place. The Republicans reelected were elected on not staying the course." Rep. Les Aspin (D-Wis.) called the same wins "a little boost. They will make people more independent of Mr. Reagan. I guess everyone is going to have a try at the compromise. It will be interesting to see how long everyone wants to work at it."

How the Smith works fit the open spaces of the gallery continued to be a topic over dinner as the 225 people sat down to a meal of Medallions Bolton Landing, named for the New York farm where Smith made all of the sculpture in the show, and Chocolate Zig, named after one of Smith's later series. "This show justifies the space of the museum," said economist Georges de Menil, of New York and Paris, who lent one of his three Smiths to the show.

The Smith exhibit is the first time the AMA has underwritten an art show and the first time the National Gallery has had a corporate sponsor that was not a corporation.

"We want to encourage artists. This is part of our social responsibility, as well as corporate investments," said Wayne Bradley, AMA vice president for public affairs. "Frankly, we are also aware of cutbacks in federal support for the arts." In its new downtown Washington building the AMA has installed a collection of contemporary art. In its lobby is a Louise Nevelson sculpture.

The $350,000 cost to the AMA was well spent, said the organization president William Rial, and would not have been better spent on medical care. "There's no question that the federal government is also cutting back on medical programs. We are responding to that in a different way by urging doctors to take particular care with the poor or those who have lost their insurance."

Other guests included Muffie Brandon, Nancy Reagan's social secretary; Martin Feldstein, the chairman of the Council of Economic Advisers; Rep. G.V. (Sonny) Montgomery (D-Miss.), members of the Smith family and artists Helen Frankenthaler and Anthony Caro. Caro, who was eating under the shadow of one of his own sculptures, remembered his friend Smith. "Once I was going to Bennington. He wanted to fly and I insisted on taking the bus. So we rode up on the bus and he had these tiny little bottles of brandy. And we drank all the way."