Defense Secretary Caspar W. Weinberger yesterday scaled down President Reagan's claim that "the Soviet Union does have a definite margin of superiority" in nuclear weapons.

Addressing defense contractors attending the American Defense Preparedness Association, Weinberger said, "the Soviets have begun to build an edge of superiority," a lesser claim than that made by Reagan at his March 31 news conference.

Several senators, including John Glenn (D-Ohio), rejected Reagan's assertion, while military professionals said privately that they were stunned that the president would make it even if he believed it. National leaders seldom say a potential adversary has the advantage before a battle is joined.

Weinberger moderated the president's statement in arguing that the United States must continue shoring up its defenses, including the nuclear ones, if it is to be able to conduct meaningful arms control negotiations with the Soviet Union.

Weinberger, Pentagon officials said yesterday, overstated Soviet capabilities when he said on Wednesday that "Soviet missiles are now more accurate than ours." When queried, the officials said that no intercontinental ballistic missile that the Soviet Union has deployed is as accurate as the latest U.S. Minuteman III missile now on the line.

In his luncheon address, Weinberger rejected the idea that the Pentagon was building weapons that were unnecessarily complicated, saying that it had no choice but to stress technical advances because of the gains the Soviets are making in sophisticated weapons.