It was supposed to be a pleasant telegenic day featuring winners at the White House yesterday. President Reagan would have his picture taken first in the Rose Garden with the winners of the New York marathon, then in the Oval Office with the winner of the Nobel prize in economics, George P. Stigler.

But the winners deviated from the scripts.

Free-market economist Stigler is a University of Chicago professor known for his advocacy of government deregulation. That sounded safe enough. But six days before the midterm elections in which the stagnating economy threatens to produce big Republican losses, Stigler shocked White House aides by declaring from a White House podium after his session with Reagan that the nation is in a "depression."

He also told the television cameras that supply-side economics was a "gimmick" and a "slogan that was used to package certain ideas."

"But that's all right," he added. "Sloganeering is one of the larger products of this town. As a thesis, it had a great deal of sense to it. Where it went too far was claiming too much for it."

As White House officials listened anxiously, Stigler said "we're in a depression now," as serious "in terms of magnitude" as the Depression of the 1930s. Just a few hours earlier, Reagan had reassured a group of small businessmen about impending economic recovery, saying "there is some sunshine on the horizon" and "we are going to stay the course."

Asked to grade Reagan's performance in dealing with the economy, Nobelist Stigler told reporters, "I am an academic economist. If I were judging him on writing an economic textbook, he'd not get an A, maybe an incomplete. I want to see what happens in the next two years."

While he spoke approvingly of Reagan's efforts to achieve deregulation, the economist found fault in retrospect with the falloff in inflation Reagan is trumpeting on the campaign trail this fall, saying it came too quickly. "We had a crash program in the first six months of this year," instead of slow and steady progress, he said.

While Reagan claims his inflation-fighting efforts have not increased unemployment, Stigler concluded differently, saying that such a rapid unwinding in prices after a decade's inflation was bound to result in "costs that are substantial."

Just a week after the administration proudly announced limits on imports of European steel, Stigler also warned that protectionism in international trade would produce only a "mild, short-run fillip" for American industry and in the long run would be counter-productive.

White House officials didn't want Stigler to go on. Deputy press secretary Larry Speakes appeared at the doorway to the briefing room and gave an aide the signal to end the questioning. Stigler, red-faced, was led off the podium.

The day's other non-event went wrong, too.

In a sun-splashed Rose Garden ceremony, the president honored winners of the New York Marathon, including Grete Waitz, who won the women's division; Linda Down, a victim of cerebral palsy who was the first woman to complete the event on crutches; and three-time winner Alberto Salazar.

"We shall go back inside now," Reagan said at the end of the ceremony, trying to avoid questions from reporters. But Salazar piped up that he wanted to present Reagan with a pair of Nike running shoes, "the same brand which I wore in winning the New York marathon." Salazar is a member of Athletics West, a Nike-sponsored Olympic running club, a company official said.

He didn't stop there. "I hope these will help him in his race for reelection," Salazar said of Reagan, who has not yet declared for 1984. "I am sure that he will be reelected president."

Asked whether Salazar had been told of a presidential intention to run again, Reagan said, "Let's not embarrass him."

"I am not embarrassed," Salazar said. "I am no economic expert, but I say, stay the course."