There is an X factor at work on the Soviet scene -- Russian nationalism. Long evident to aficionados in a wriggly and diffuse form, it has taken on a new and broad political visibility with the Soviet invasion of Afghanistan and Alexander Solzhenitsyn's new article in Foreign Affairs. It is no longer possibly to see where the Soviet Union may be going without trying to pin it down.

Russian nationalism is the feeling of special identity and pride held by the ethnic or nationality group that is the largest (about half the population) and the politically most significant of the hundred-plus such groups in the Soviet Union. Through most of Soviet state's 63-year history, the ruling communist minority played it down, emphasizing instead Marxism -- which supposedly transcend ethnic differences -- as the official ideology.

The trouble is that nobody much believes in Marxist ideology in the Soviet Union any more. It is corrupt, hollow, unusable. Into this vacuum has flowed Russian nationalism.

Here things get interesting. Two different groups have turned to Russian nationalism. Their struggle to mobilize it and monopolize it is what politics in the Soviet Union is now increasingly about. This struggle transcends the "public" conventional struggle for the succession to the ailing Leonid Brezhnev: the ideas of the next generation of Soviet leaders will count for more than their names.

One group waving the Russian national banner comes from the communist elite. People in this group -- in the media, in the party, in the Politburo (crusty old ideologist Mikhail Suslov, for instance, is regularly first on his feet for the national anthem) -- see at least two virtues in it. It's more popular, more useful to legitimize their own power than Marxism. And it includes a set of ideas -- pride, uniqueness, patriotism, paranoia, grandeur, power, expansion -- compatible with much of the Kremlin's current foreign policy. Imperial bluster gives a mission abroad too a leadersship with nothing in particular to champion at home. Hence Afghanistan.

The second group starts with Alexander Solzhenitsyn and includes some hundreds or thousands or, conceivably, millions of others. Totalitarianism makes it hard to count, but the evidence -- accounts of exiles and emigres, church activity, Kremlin irritation -- suggests the substantial dimensions and potential of this opposition.

It not only can draw on the same pool of traditional ideas that feed the official Russian party; it also can draw on the moral authority earned by the immense suffering of the Russian people under communism. Hence Solzhenitsyn's threat to the Kremlin; hence the Kremlin's underlying love-hate attitude toward Solzhenitsyn.

Well, you may say, this is a tangy as pickled mushrooms to old Moscow hands, but what difference does it make if a Soviet invasion of a neighbor is inspired by the class struggle or nostalgia for Ivan the Terrible? One answer is that, in the first instance, the fellow travelers of the world will go along with Moscow and, in the second, the nationalists of the world will not. Still, the risks in the new Kremlin mix are apparent. It does make a difference that the Kremlin, having lost its of zeal for the export of revolution, acquires a new zeal simply to throw around its power -- since it now has more power ever before.

A bitter argument -- a battle for the soul of Russia -- is now raging between Solzhenitsyn and his many critics, especially among emigres. Solzhenitsyn contends that Russian nationalism is essentially a humane, ethical and religious (Russian Orthodox) movement. They find him fronting for an authoritarian, potentially fascistic, anti-Semitic nativism. Russian history nurses both strains.

Teh West has its own stake in how this argument comes out. The inward-looking slovophile aspect of Russian nationalism is at heart less congenial to the West than the Westernizing democratic movement that has furnished most of the dissidents the Western public has saluted in recent years. But the democratic movement is frail and thinly rooted in Russian soil.Will nationlism go the humane anti-Stalinist way pointed by Solzhenitsyn in Foreign Affairs or the way of primitive neo-Stalinism? The West is bound to favor the first road.

Finally, Solzhenitsyn insists that his people, the Russians, do respect other Soviet nationalities. Much history demonstrates otherwise. But in a country (and age) charged by ethnicity, it seems certain that Russian nationalism will further arouse the nationalism of other Soviet ethnic groups. The communists who encourage it to keep themselves in power, and no less the opposition figures who encourage it to overthrow communism, are riding a tiger.