The president had 1,000 business leaders over for drinks and a pep talk in a September rain yesterday, all heads covered by a soggy South Lawn tent. It had leaks not nearly so troublesome as Wall Street's reaction to the Reagan economic program.

Ronald Reagan never mentioned Wall Street by name, but no one missed his intent. "Suddenly we hear some saying that our programs aren't working," he said. "Well, of course they're not. They don't start until Oct. 1."

And earlier: "For those doubting Thomases, I have a message. They're wrong -- again."

The business suits applauded and cheered, a partisan crowd enjoying a very choice fruit of labor: an invitation to the White House, a thank you for pushing the president's economic program. "You're the key to the economic recovery of this nation," Reagan said, more tanned and rosy-cheeked than either budget director David Stockman or Treasury Secretary Donald Regan. Both were there.

After threatening to veto appropriations bills that come in over budget ceilings and after finishing his accolades to the business leaders, Reagan got to the gist: "There are many trials yet ahead," he said. "I hope we can count on your help . . ."

Most likely they can. This was a crowd of fervent Reaganites -- from mom-and-pop store owners to General Motors lobbyists -- ready to push for round two of the budget cuts. Many said that Wall Street's fears of deficit and inflation don't frighten them. Others were mad.

"Wall Street is sitting in the catbird seat," said Arthur Fletcher, a Washington consultant who ran unsuccessfully for District mayor in 1978. "I'm of the opinion that Wall Street has a contribution to make, too."

Some were more cautious. Asked if he had complete confidence in the economic program, Pete McCloskey of the Electronics Industries Association replied carefully: "That's a very tough question. I'd be confident if the confidence of the people is rekindled."

The reception, despite the economic overtones, sounded something like a party under the Big Top. A Marine band played splashy tunes, giant poles held up the white canvas tent, guests nursed their soda waters. Many walked good-naturedly around the plastic buckets that were placed to collect the drips. Muffie Brandon, White House social secretary, quietly endured the rain that stained the back of her rust-colored silk dress.

Reagan, who signed his economic recovery package in a California fog, had to compete with a downpour as he pushed the program yesterday. The shower began shortly after his remarks aimed at Wall Street. There was a low rumble from the heavens, and then the clouds opened.