Russians make up only half of the population of the Soviet Union, and foreign observers have often wondered whether the other half will always put up with their rule. The spark that may ignite a national rebellion against Russian domination by one of the subject peoples, if it ever comes to that, it most likely to come from Soviet Georgia, whose students have just won a notable victory against the Communist Party.

The new constitution, which sought to replace Georgian as the sole official language of the Georgian Republic, provoked a student march on the Parliament building in Tbilisi - and the Communist Party leadership promptly caved in. Troops had been called out, and it is quite possible that if the students' demand had not been immediately accepted there could have been fighting in the streets, loss of life, widespread disorder throughout Georgia, and a major rebellion whose consequences would have been difficult to foresee.

Many of the Soviet Union's non-Russians fear Russification like the plague, and they regard any slight to their own languages as a step on a road that could deprive them of their national identity. This is an issue in each of the Soviet "national" republics. Even in the Ukraine, whose Slav language shares a certain affinity with Russian, the rise of nationalism has brought the language issue increasingly to the fore.

In some ways the language struggle has become the symbol of the resistance to Russian rule, because on the surface, at least, it is politically the less sensitive issue. It is safer, for instance, to take up the cudgels in behalf of the purity of one's language, to protest the infiltration of Russian words into it, than to protest the infiltration of highly paid Russian bureaucrats and technocrats into the Ukraine, or Georgia or any one of about a dozen other republics. But rule by outsiders, whether it is exercised directly from Moscow or by Russians in key jobs in the various republics, is often the real issue.

In Georgia, whose history and culture are much older than those of Russia, national pride combines with a fiery national temperament to make the language issue into something more than a symbol. At the Georgian Writers' Congress two years ago, one speaker, Revaz Djaparidze, used the language issue to denounce the "chauvinist" view that "every nation should be subordinate to the Great Russians." He cited the views of certain Russians who wanted "to ban completely the inferior language spoken by 60 percent of Russia's population." He had dug up a speech of Lenin, who had once denounced Russian chauvinists, and he merely quoted from it - to the delight of his audience.

But he also protested against attempts by the Ministry of Education to start some schools in which, after the fourth year, children would be taught exclusively in Russian. He say it as the thin end of a wedge that might lead to the Russification of the school system. Tbilisi University, he said, had just announced that some courses would be taught only in Russian. On orders from the Moscow, some university textbooks in Georgian had been replaced by Russian books.

The Georgian party secretary, Eduard Shevardnadze, who attended the writers' congress and who tried to answer some of Djaparidze's complaints, was repeatedly interrupted from the floor by writers who shared Djaparidze's misgivings. But Shevardnadze was to get his chance to make a reply in the more controlled environment of a conference specially convened by Tbilisi to denounce the nationalist menace. He attacked those who used the language issue to "slander" the party. They represented themselves, he said, as defenders of the "national" language and "national" schools. "There is no room in our society," he said, "for self-appointed pseudopatriots who raise a hullabaloo to grab cheap glory and cheap popularity for themselves."

The nation, he insisted, had no need of their "services" - but at the same time he made it obvious that he was concerned about the following they had secured in the nation. There had been relapses, he said, into "narrow nationalist interests" in such fields as literature and art, science and culture - and "they have not always been properly repudiated." The party's own stalwarts, those who believed in the Russian connection (he called them "internationalists") were themselves being denounced, he said, as "poor patriots."

What is all boils down to is the growing feeling against Russian domination in the Soviet Union's "national" republics. Few countries are more vulnerable to the demand for autonomy and independence than the Soviet Union, with its patchwork of nationalities ranging from the Ukraine, with its population of 50 million, to Georgia, with nearly 5 million. What's more, the nationalists in the Soviet Union can find support in Lenin's old writings for their demands - and they use them, to good effect.

As Shevardnadze told his party audience, "certain people who have become enmeshed in the coils of petit bourgeois nationalism" were using Lenin's teaching one-sidedly. They saw in it only what Lenin had said about a nation's right to self-determination and its right "to split off into an independent state." That is obviously what some Ukrainian nationalists are demanding. What we do not know is how and when such demands could present a direct threat to the Communist regime.

But if the Tbilisi language demonstrations had turned into a riot, some such demands might easily have come to figure on the demonstrators' banners. It was Shevardnadze, in his position as the Georgian party's first secretary, who accepted the demonstrators' demands and announced that Georgian would remain the republic's only official language. He obviously knew that worse might happen if he failed to give in.