Election landslides often are followed by great misadventures, as presidents allow themselves to be misled as to the meaning of their mandates.

In the past half century there have been four elections in which incumbent presidents won electoral landslides that approached or exceeded 60 percent of the popular vote.

In 1937, after a reelection victory in which he carried all but two states, Franklin D. Roosevelt unsuccessfully tried to enlarge the Supreme Court, then busy striking down New Deal legislation. This court-packing scheme, which was met with charges that Roosevelt was behaving like a dictator, carried over to the 1938 midterm elections, where FDR failed in attempts to purge eight conservative senators.

In 1965, the year after he won a record 61 percent of the popular vote, Lyndon B. Johnson used his mandate for peace to widen the Vietnam war, with tragic consequences.

And in 1973, flush with the success of carrying everything except Massachusetts and the District of Columbia, Richard M. Nixon directed a Watergate cover-up that ultimately drove him from office.

Of these three presidents, only Roosevelt kept his good reputation, largely because the events of his second term were subsumed by his achievements as a wartime president.

Now it is Ronald Reagan's turn. While he has been savoring his easy victory at his mountaintop ranch in California, storm clouds have been gathering over his administration.

The candidates for foreign policy misadventure are many, with Central America probably leading the list. Beyond any calculation, the reality of worldwide terrorism has added new unpredictability to the calculus of foreign policy.

However, though he may not understand the inner workings of an intercontinental ballistic missile, Reagan has learned some of the lessons of Vietnam, as he demonstrated by withdrawing the Marines from Lebanon after the catastrophe in Beirut. The president would like a different government in Nicaragua, but he recognizes that the American people are firmly opposed to using U.S. combat troops to achieve that end.

On the face of it, Reagan faces a danger different from that of other landslide winners. It is a danger less of adventurism than of drift and miscalculation by indecision.

Consider what has happened in the three weeks since the election. Reagan's aides had planned "a unity tour" featuring historically symbolic speeches at Plymouth Rock in Massachusetts, Gettysburg, Pa., and Kitty Hawk, N.C., and politically symbolic meetings with black groups and the defeated Democratic ticket of Walter F. Mondale and Geraldine A. Ferraro.

The impact of this tour, which was scrubbed because the president's aides could not agree on its form and substance, would have been to reinforce the impression of Reagan as a truly national leader seeking to address the nation's serious economic problems in a bipartisan fashion.

Instead, Reagan seems to have lapsed into a period of semi-retirement that contrasts sharply with the way he hit the ground running after winning the presidency in 1980.

While Reagan has relaxed and the factions within his administration have taken battle positions, the economy has slowed and unemployment has edged upward. Increasingly, Reagan's rosy reliance on a 4 percent annual growth rate seems an illusion.

The president also has underscored his campaign opposition to tax increases and cuts in the defense budget or Social Security. Small wonder that Wall Street is jittery and that the market is behaving as if Mondale had won.

If Reagan holds to his promises on taxes, defense spending and Social Security, and honors his pledge to make a serious attempt at reducing the deficit, he will have to cut remaining social spending programs in a Draconian fashion, perhaps by as much as one-third. A budget attempting this would be dead on arrival at Capitol Hill -- or "dead on leak," as some administration officials put it.

If Reagan doubts that Congress would have substantial public support in resisting such a budget, he should read beyond his mandate of Election Day. For instance, he might consult a post-election Gallup Poll showing that Americans overwhelmingly support increased spending for social programs while, by a narrower margin, favoring cuts in the defense budget.

Reagan wants very much to be a successful second-term president, but he needs more than good intentions. He needs to emerge from the euphoria of his mandate and make some hard choices among his conflicting campaign promises.

The question before the Democratic-controlled House, not to mention the GOP-controlled Senate and the country at large, is whether Reagan is up to this kind of decision-making. Can he make the hard choices, even if it involves acknowledging that he overpromised? And can he make them in time to avoid the economic storm?