A leading student of the American presidency says Ronald Reagan might have steered clear of the Iran-Contra scandal if he had the advice of the person he most depended on to warn him of trouble: Nancy Reagan.

The former president was handicapped by lack of experience in foreign policy and lack of curiosity about details, political scientist Richard Neustadt writes in a new edition of his 30-year-old classic, "Presidential Power.""Still, it may be that his greatest lack was not experience, direct or indirect, . . . but rather anything to activate the aide in charge of warning him when threats appeared against his public standing or historical appeal," Neustadt says.

"That special staff role, of immense importance to someone habitually incurious about detail, had been assigned to his wife. More precisely, she had made it hers since Sacramento." Reagan began his political career as governor of California.

In the Iran-Contra affair, Neustadt argues, Nancy Reagan could not play her customary role of adviser because of the secrecy in which the arms-for-hostages operation was carried out under the supervision of the late CIA Director William Casey.

Reagan, he says, relied on Casey "in such a way as to cut Nancy off."

"Never let your Nancy be immobilized, could be a rule of thumb for future presidents," the presidential scholar writes.

Neustadt's description of Nancy Reagan's role as an adviser contrasts with that of fired White House chief of staff Donald T. Regan, who revealed in his memoirs that some of her advice, on matters of the president's schedule, depended on astrology.

This, Neustadt concedes, was "silly, to be sure, though possibly no more so than the whims of staff, or press, or partisans, or family quite traditional in such decisions."