President Reagan plans to curtail the once-sweeping congressional activities of Office of Management and Budget Director David A. Stockman, with Treasury Secretary Donald T. Regan becoming his most visible spokesman and salesman of economic and budgetary policy, congressional and White House sources said yesterday.

The move is being made, the sources said, because of continuing private concern about Stockman's credibility in the wake of his quoted criticisms of the administration's economic program in the latest issue of the Atlantic Monthly magazine.

The decision came after Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) had reported to Vice President Bush that his informal discussions with about a dozen top Senate Republicans and Democrats showed a widespread belief that Stockman had "seriously damaged" his effectiveness, particularly when it comes to future congressional testimony on the budget, according to several sources.

"Donald Regan is moving to the forefront," said one Republican congressional source. "Regan will be the point man, the one who comes to the Hill and testifies on the big picture. And Stockman, when he does testify, will be more of a detail man--someone to explain the numbers."

A knowledgeable White House official added:

"Stockman will continue to testify at budgetary hearings. That has to be. But there's no doubt that Don will be out front--at least for the forseeable future. Dave's visibility will be lowered. He will be assuming a lower profile."

For Stockman, a lowered official profile is better than none at all. And at the White House, the president and his senior advisers are still waiting for the initial tremors touched off by the Stockman quotations to subside before making a final assessment as to just what role Stockman should play in the public selling and congressional negotiating of the Reagan economic policy.

According to a congressional source, one of the president's most senior advisers offered this assessment of the White House's approach to the future of the budget director:

"We're in a posture of damage control now. We're trying to buy ourselves time to see how it plays. We've got to see if this hurts his Stockman's effectiveness."

And a senior White House official, when this assessment was read to him, said it seemed an accurate reflection of the situation at hand. "I have no quarrel with that," he said.

For the first 10 months of the Reagan presidency, Stockman was the dominant figure in the shaping and selling of domestic policy, especially when it come to forging compromises in Reagan's package of sharp domestic program cuts, defense spending increases, and continued tax cuts. Now, White House officials said, "it remains to be seen as to whether Stockman will have a major role in the negotiations."

The White House has received word from Baker that Stockman may have suffered lasting damage. Last Thursday, just before the president met with Stockman, Vice President Bush asked the Senate majority leader to let him know what the feeling was among influential senators.

Baker talked to about a dozen senators, Democrats as well as Republicans. At about 1 p.m., according to congressional sources, Baker telephoned Bush with a negative assessment. Said one source: "The senator told the vice president essentially this--'The incident has seriously damaged Stockman. It has hurt his credibility and effectiveness, particularly when he has to go up to the Hill to testify.' "

Just what Bush did with this information is not clear. Through a spokesman, the vice president denied that Baker had ever given him any assessment. But Hill sources confirmed that in fact Baker had.

The president allowed Stockman to retain his job, but withheld expressions of personal support or confidence. Replacing this highly knowledgeable budget director in these final months of shaping next year's budget would have been a difficult, and probably self-debilitating ordeal for the president and his policies, one senior White House official said.

"What happens in the future remains to be seen," the White House official said. "Right now his visibility is being lowered. Dave himself believes this is a good idea."