Sen. Edward Kennedy has taken an undoubted step toward a presidential candidacy in 1980. But as he advances he brings in train a series of questions -- questions that have to be put, and which he needs to answer, if the country is to determine whether he can be a good president.

First, there is the matter of expectations. The last four presidents have come to grief because expectations exceeded what they could deliver.

Lyndon Johnson raised hopes of peace and progress and, in the eyes of many Americans, failed on both counts. Richard Nixon did not live up to his promise to bring the country together, or even to settle the war in Vietnam, Gerald Ford, because he did not originally campaign for the office, was able to underline his limitations. "I'm a Ford, not a Lincoln," he used to say. But expectations run so high that his candor yielded the general opinion that Ford was not bright enough to be president.

As to Jimmy Carter, the storm over Cuba is only the latest development to mock his claim to be a competent manager. Though he seems transparently moral to many, disbelievers have had a field day with such insignificant matters as the visit of Hamilton Jordan to Studio 54. So by promising to be morally good, Carter has made it possible for mischiefmakers to do with the charge "cover-up" what Red-baiters did with the "soft on communism" charge three decades ago.

If only because he is the brother of his brothers, Kennedy outshines all other public figures as a focus for hopes and aspirations. So it is a genuine question whether he can meet the first challenge of the post imperial presidency -- the challenge of lowering expectations.

Then there is the challenge of bringing the country together. Rightly or wrongly, the senator is identified with all the great, but unwon, causes of the '60s -- gun control, busing and a more tolerant attitude on such matters as abortion and drugs. He is also identified with extreme positions on the most vibrant current questions. In foreign policy, he seems to lean to the view that the United States should cut back on defense, move forward on arms control, and bend to the pressures of the neutralist countries of the Third World.

He also seems to favor an activist role for the federal government in managing the economy and providing social services -- particularly in the health field. In the matter of energy, he at least gives countenance to the argument that the shortages and high prices can mainly be blamed on dirty work by the oil companies.

If all these assumptions are correct, if Kennedy is actually hooked on far-out views as a matter of deep and unremitting conviction, then his prospects for leading the country effectively are dim indeed. On the contrary, he will raise even higher the expectations of the liberal constituency -- particularly in the Northeast and California.

By the same token, he will raise hackles in the South and most of the West. Divisions will be prolonged and intensified. The new conflict will breed more frustrations, and another round of deepening cynicism.

But the senator is, after all, a practical, working politician. It is possible -- even likely -- that he came to many of his present views as a representative of Massachusetts or the paladin of the liberal bloc in the Democratic Party.

It seems reasonable that he would make adjustments as he expands his horizons. As a national candidate, he would presumably want to let lie the sleeping dogs of the 1960s. He would at least respect the constituency that believes the country needs to strengthen its defenses against the Soviet Union and stand up to Russia's pawns in the Third World. Above all, he would accommodate to the majority view that the federal government is poor at managing the economy, particularly bad at providing services and downright awful when it comes to allocating supplies of gasoline.

If he can bridge the gap with the majority, then the senator can wind down the unrealistic expectations so characteristic of his liberal constituency. He can put together the coalition necessary for cohesion in the Democratic Party and Congress. He can be a genuine leader.

So in the weeks and months ahead, it will be important to scrutinize the stands of Sen. Kennedy. It will be interesting to see whether he has the skill to translate popularity into the more solid stuff it takes to govern this contentious country.