The Soviet Union is in the throes of what the Moscow newspapers describe as a "nationwide debate." Every day Pravda, which usually runs to about six pages, gives up a whole page to reports of the debate, with accounts of meetings being held in various parts of the Soviet Union, letters to the editor and specially written articles, all on one subject: the new Soviet constitution.

So far, however, not a single voice has been rasied in protest against any of the provisions of the new constitution. The meetings being held in each of the 15 republics that make up the Soviet Union pass resolutions that "unanimously" approve the draft of the constitution recently unveiled in Moscow by Party Secretary Leonid Brezhnev. But the speech made on the subject of Brezhnev himself did contain some hints of differences in the leadership.

The Soviet Union is formally a "federal state, with each of the republics retaining certain constitutional rights. The rights of a republic to secede from the Soviet Union, which was written into the Stalin constitution of 1936, has always been an empty formality, as have been most of the other "rights." But the nationalist sentiments that have come to the surface from time to time in the various Soviet republics must have the Kremlin wondering whether the right to secede should be retained. Reports reaching the West from the Soviet Union have indicated that this was one of the subjects under discussion during the drafting of the new constitution.

There are evidently three schools of thought in the Soviet Union. There are those, mainly in the republics, who believe that the federal element in the structure of the state should be greatly strengthened, to allow for a real degree of devolution from Moscow. They may be rougly compared with those in the West, such as the Scots in the United Kingdom or the Basques in Spain, who believe, for historical and economic reasons, that they ought to govern themselves.

Then there are those, mainly in Moscow, who believe that an even greater degree of integration is required than exists already, to prevent the emergence of powerful nationalist feelings that might threaten the Kremlin's control of the non-Russian republics.

The third school of thought evidently holds that the whole subject is potentially so explosive that it is best to leave well enough alone.

It is certainly a matter of great political sensivity, and few details of the Soviet leadership debate on it have been allowed to percolate into the public domain. But even some members of the Politburo have on occasion argued in favor of greater devolution, as became evident after the Ukrainian party secretary, Pyotr Shelest, was dimissed from his post. The first party secretaries of such important republics as Armenia, Georgia and Azerbaijan were also dismissed in recent years amid indications that they had inclined to a "national Communist" viewpoint that favored the interests of their own republics. Translated into constitutional terms, this means that they and others like them believe that the federal element in the structure of the Soviet state should be strengthened.

It was this debate that Brezhnev seems to have been hinting at when he said recently that experience had shown that the main features of the Soviet federal structure had "fully" justified themselves. "There is therefore no need," he argued, "to make any changes of principle on the forms of the Soviet Socialist Federation." As is so often the case in Soviet leadership debates, it is only deduction that one may conclude that someone had been arguing in favor of changes. Otherwise, there would have been no need for Brezhnev to reject the idea of change.

One of the suggested changes, to judge from an article that appeared in the Soviet press last year, was that the new constitution replace the federal structure of the state with a unitary one, so that the republics should be controlled in law, as well as in fact, from Moscow. The article recalled that Lenin regarded the federation as a "transitional stage" on the way to full unity. The time might come, the article suggested, when the federal structure of the Soviet state might hamper the "coming together" of the nations that compose it. This, it said, would raise the question of transition from a federal to a unitary state - a prospect the article warmly commended.

Actually, no such changes were made in the new constitution, presumably because who believed that it is better to let sleeping dogs lie had won the day. But does it matter what the constitution says? On some issues it does matter. Obviously no republic could secede from the Soviet Union by just declaring its wish to do so, even though the provision that allows this has been carried over from the Stalin to the Brezhnev constitution. But if the first secretaries of some of the more important Soviet republics were again to develop nationalistic leanings, they might demand that the rights "guaranteed" by the constitution be honored.

The new constitution has abolished, for instance, the right of each republic to raise its own military units. No republic has made any use of this right any more than the right to secede. Why, then, should the first be dropped and the other retained? Evidently because the Kremlin feared that in certain circumstances nationalist agitation might lead to demands that some of the republics be allowed to form their own military units.

In the end, however, it is not the constitution but the facts of life that will weaken the dominant position of the Russians in the Soviet Union. Russians now constitute little more than half the population of the Soviet Union, and their birthrate is declining, while that of the non-Russians, particularly in the Asian part of the Soviet Union is rising rapidly. It is already that by the 1980s some Soviet military units will consist predominantly of non-Russians. It is easier to write new constitutions than to change demographic trends.