The question of Leonid Brezhnev's departure from power, which administration officials have long been reluctant to discuss in public, is beginning to find its way into officials' statements. There is a quickening of interest in what will happen after Brezhnev, a feeling that Soviet policy may be coming to a crossroads and a belief among some officials that the United States should act in a way that would promote the changes it seeks. But there are also objections to doing anything of the kind.

The "wholesale generational turnover" about to occur at the upper levels of the Soviet power structure has now been discussed before a congressional committee by Marshal Shulman, the Soviet affairs adviser to Secretary of State Cyrus Vance. He believes that the Soviet Union is now on the threshold of a change of generations, though this need not occur immediately after Brezhnev's departure, which may be followed by a transitional regime. But, he argues, it is clear that an ascendant generation will be holding the levers of power within the foreseeable future, and he wants us to be ready for it.

The succession issue is only a small part of Shulman's statement, which provides the most comprehensive account of the administration's view of the Soviet Union, and of U.S. relations with it, since President Carter took office. But it is a crucial issue, and in a recent conversation Shulman discussed some of the factors affecting it. He believes that two contradictory tendencies constitute what he calls the "central drama" of Soviet political life.

One tendency is toward orthodoxy, and the other is toward modernization. They find expression not only in the existence of different factions, he says, because sometimes an individual experiences the pull and the push of both these tendencies. There is also a strong nationalistic current, favoring the isolation of the Soviet Union, which entails an autarchic, self-sufficient economy. He discerns its origins in the centuries of centralized, bureaucratic rule that prevailed under the czars.

But even then there was a conflict between the Slavophiles and the Westernizers, and he believes that a similar conflict is discernible now - and that the coming generation may well be more outward-looking. The people in the 40-45 age group are technologically well educated, have a high degree of competence and are equal to comparable groups in the West - which is less true of the present generation of leaders. The major task they face on assuming power is the rationalization of the economic structure - and there are those, he says, who look to Western models in their efforts to modernize the Soviet Union.

Shulman delves into the past for pointers to the future. He recalls that Khrushchev tried to modernize the economic structure and found the resistance too powerful. The 1965 exonomic reform launched by Prime Minister Alexei Kosygin was similarly thwarted by opposition from the bureaucrats and from orthodox party officials who resented new ways of doing things and feared that their careers would be jeopardized.

On the other hand, "It is very clear that Soviet economists have known what needs to be done," that they knew it then and they know it now. The highly centralized system of political control is responsible in part for the Soviet Union's low productivity and for the fact that it lags behind the West in advanced technology, but this does not mean that the new leadership generation is bound to relax the controls. "We just don't know what they will be like," whether they will move forward to Western-style modernization, or back to nationalism and orthodoxy.

What Shulman is concerned about is that if people come to the top in the Soviet Union who want to move forward, they should not find the road closed by us. If they want to put the stress on economic modernization, then for them the prospect of trade with the United States, of acquiring advanced technology, "would be a major factor." If they want their country to become involved in the world economy and the world community, "we should not slam the door in their faces."

But Shulman's critics argue that he is, in effect, advocating concessions to Moscow for the sake of an uncertain return. They maintain that the prospects of the succession to Brezhnev are so much in doubt, the course of the power struggle so uncertain, that it would be the height of folly to make the shape of Washington's policy depend on possible developments in Moscow. Shulman is not suggesting that the West should intervene in the power struggle, but at a time when the succession is at issue, the matter was bound to come up. Can the West intervene effectively?

The question was addressed by a State Department analyst, Sidney Ploss, who took part in the panel on Kremlinology at a recent conference in Washington. He recalled the U.S. actions that contributed to the fall of Khrushchev, citing the U2 incident of 1960, and spoke of other miscalculations in U.S. policy, which he ascribed to the fact that "signs of the Kremlin power struggle were not taken seriously" in Washington. Relations with Moscow cannot be planned intelligently, he believes, if the planning fails to take account of the way in which Western policies affect the struggle in the Kremlin.

Indeed, he suggested that a study of how such moves affect the Kremlin power struggle should be written for American leaders. It is certainly a study from which President Carter could benefit greatly as he begins to grapple with the problem of the succession to Brezhnev, and of the policies that the United States should follow as the post-Brezhnev regime begins to take shape. Those who favor Shulman's argument and those who oppose it might then have something more than their opinions - or prejudices - to back their stand.