The impulse to make a final definitive pronouncement on Communist power, one that will provide once and for all a clear, agreed basis for judgment and policy, is as old as the Communist revolution itself. President Reagan yielded to it when he asserted that the Soviet Union is a cynical revolutionary state bent on expansion in the name of Communist ideology.

But Reagan was almost certainly wrong in his assertion. He was wrong not so much in the particulars -- the Soviets are cynical, are revolutionary, are bent on expansion, are beholden to Communist ideology -- as in his overall view. For though the Soviets are all these things, they are also other things. The others increasingly count more.

The others are: Russian nationalism and chauvinism; the exaltation of power (especially military, including technological) and ethnic arrogance, meaning Great Russian racism directed at the 100 other ethnic or nationality groups in the Soviet Union and at China.

The ideology is not formally dead, as we will see at the Soviet Communist Party congress starting Monday. But most students of Soviet power would agree, I think, that the ideology is not all that dominant, either as the driving force of Soviet policy at home or as the medium in which the Kremlin can reach out to people abroad.

Behind Soviet power, more and more, is Russian nationalism. That is the element on which the Kremlin relies to appeal to Soviet masses turned off by ideology and to provide a rationale for its military surge of the last 20 years. I note that an especially keen student of Soviet affairs, Alexander Solzhenitsyn, has chosen Russian nationalism -- in, he promises, a benign form -- as the horse he intends to ride to political power in his homeland power in the not too distant future.

Does it matter whether the guiding intelligence of Soviet power is ideology or nationalism? Need anyone beyond the community of Kremlinologists -- a tendentious lot -- care?

Yes. Why, for instance, would the president himself want to certify that the official Soviet ideology is a live and potent (if dangerous) force? It's false flattery and free advertising. It portrays the ideology as a train moving through history and invites power-seekers of a certain sort to hop aboard. Why promote the notion that two coherent ideologies are striving? We should not surrender so casually our pride and political advantage in possessing what truly is a live and potent ideology -- respect for freedom and diversity.

Many foreigners will judge us, rightly, by our perception of Soviet power. If they thing Americans are in the grip of an outmoded perception, their confidence in American policy will falter accordingly. It is going to very tough anyway for the new administration to get the allies in line. The president strews rocks in his own path by offering a conception that gives important constituencies in Europe and elsewhere an intellectual pretext to opt out. What they want, and what we deserve, is a more flexible conception, one that provides a basis not only for vigilance but for taking up what opportunities for East-West cooperation may yet arise.

In the hands of the current Kremlin leaders and (one guesses) of the younger military-minded technocrats pushing up behind them, Russian nationalism is an ugly thing. But it has its weaknesses. It will invariably nourish Ukrainian, Asian and other ethnic movements within the Soviet Union, widening precisely those cracks that earlier Soviet leaders sought to bridge with an overarching Communist ideology. Russian nationalism, moreover, is bound to stir countering local nationalisms in foreign places -- Afghanistan and Poland are good examples -- where Kremlin power is brought to bear.Nor can the "natural" foreign leftist partners of the Soviet Union be easily enlisted in its service. In brief, there are tactical openings.

Finally, there is a strategic question, suggested by, among others, emigres Elena Klepikova and Vladimir Solovyov in a forthcoming book called "Russian Paradox" and by old Moscow hand Harrison Salisbury in The New York Times Magazine of Feb. 1. Aficionados of Russion nationalism, they see the possiblity of some sort of great (bloody) unspecified convulsion born of the failures of the system, the consequent domestic unrest, fear of China and the thrust of the new Russian-national "party."

This is, I grant, pretty far out. It is not the kind of development the United States can sit down and plan for. But it is something for a new administration to think about as it weighs the places and times and manners in which it may try to check Soviet power.