It seems that there were two important factors in President Reagan's 49-state election sweep last year: carefully listening to the advice of the "gray eminence" Richard M. Nixon, and getting First Lady Nancy Reagan off Air Force One. This new campaign autopsy, "The Quest for the Presidency 1984" is contained in a book by Newsweek senior editors Peter Goldman and Tony Fuller.

The Bantam hardback, which will be in bookstores this week, reveals that Reagan's political advisers secretly cultivated Nixon. The disgraced former president was flattered by the attention and turned out to be correct in his prediction of Reagan's overwhelming victory. He considered Democratic candidate Walter Mondale "a dull clod" and the weakest candidate the Democrats could field. At a secret meeting at Nixon's Saddle River, N.J., home, the authors write, a decision was made to target a Mondale must-win state and as a result $4 million was pumped into the campaign in Ohio. Nixon also predicted that New York Gov. Mario Cuomo is the Democrats' best bet in 1988.

The writers also report that Nancy Reagan was tactfully bumped from Air Force One because it was believed her nonstop worrying caused the president to tighten up and become isolated. Longtime Reagan political adviser Stuart Spencer was assigned the task of suggesting to the first lady that she would do more good (by implication less harm) if she campaigned separately.