The hottest theme in our political culture at the moment is the inevitability of American decline. The current issue of Foreign Affairs (still the closest thing we have to an official Establishment voice) is largely devoted to it. So is the cover story in the current issue of Newsweek. But the most extensive and formidable development of this theme is a book called ''The Rise and Fall of the Great Powers" by Paul Kennedy, a professor at Yale.

Neither the subject matter of Kennedy's book nor its point of view has anything in common with Allan Bloom's ''The Closing of the American Mind.'' Yet the two are often compared because they are both long and difficult works that have surprised everyone by becoming best sellers.

As it happens, the liberal Kennedy thus far shows no sign of attracting as many customers as the conservative Bloom has done. Therefore, if we were to take their respective sales as a kind of poll pointing to deeper stirrings of the public mood than the usual survey techniques can chart, we would have to conclude that conservatism is leading liberalism by a comfortable margin.

The problem with such an interpretation, however, is that it fails to distinguish between two different sets of issues.

There can be no doubt that the astonishing appeal of Bloom's attack on liberal relativism reveals a very widespread uneasiness, even among liberals, over the destructive effects of the liberal culture, especially on the young. But the yearning so many Americans feel for greater moral and intellectual certainty in their personal affairs does not seem to extend to their ideas about foreign affairs. In that realm, liberal relativism has been growing stronger rather than weaker. And here is where Paul Kennedy comes in.

To Kennedy, America represents nothing special in history, nothing of any permanent or universal validity or value. The United States, as he sees it, is just the latest in the long succession of countries that achieved imperial preeminence and then lost it by overextending themselves.

If the United States stands for nothing that can be called good, neither does the Soviet Union, in Kennedy's perspective, represent anything that can be called evil. Nor are the Soviets much of a geopolitical threat, since they suffer even more profoundly from ''imperial overstretch'' than we do.

What follows from Kennedy's analysis is that we can beat a strategic retreat without endangering our security. Indeed, he says, it is only by pulling back that we can deal with the real threat to our security, which is not military but economic and comes more from Japan than from the Soviet Union.

This is all pretty familiar stuff. But that may well be the secret of Kennedy's success. For what he has donein this huge survey of ''economic change and military conflict from 1500 to 2000'' is to put all hislearning and his great gift for intellectual synthesis entirely at the service of the conventional liberal wisdom concerning America's role in the world.

In making this mountain, Kennedy has understandably earned the gratitude of all who hold property rights over the molehill out of which hehas made it. Yet neither a bibli-ography that would take most of us two lifetimes to get through nor 1,200 or so footnotes can cover for the weaknesses of the underlying argument.

Obviously the United States is no longer as powerful, relatively speaking, as it was in the aftermath of World War II. We did not need Paul Kennedy to tell us that. But has our decline really been caused by the economic factors that Kennedy and so many others keep harping on? What, for example, do economic factors (or, for that matter, ''imperial overstretch'') have to do with the defeat we are now inflicting upon ourselves in Central America?

Nor is it true that a nation's power is necessarily a function of its economic and technological resources. As the case of Japan demonstrates, a nation can be economically strong and yet lack all other forms of power. As the case of the Soviet Union shows, a nation can be economically weak and yet command overwhelming military and political strength. And as the case of the United States in the 1950s proves, a nation can enjoy preeminent power in every field and yet shrink from imposing its will on either its friends or its enemies.

Kennedy knows all this. Nevertheless, even while repudiating economic determinism, he still places the decisive emphasis on economic forces. One reason, I would guess, is that such an emphasis gives an air of inevitability to the processes he is describing. By contrast, a stress on political, moral and ideological forces would leave much more room for the operations of free choice. And that, in turn, would contravene the suggestion that further retreat by the United States is in effect dictated by history and nature alike.

There is also a more narrowly political element at work here. As E. J. Dionne Jr. of The New York Times shrewdly put it just the other day: ''For Democrats, the 'end of empire' idea has the benefit of redefining the issue of world power in economic terms, which conveniently moves the discussion away from military strength, an issue on which the Republicans have tended to have more credibility.''

Unfortunately, the appeal of ideas like Kennedy's is not limited to liberals and Democrats. On the contrary, similar notions have already provided an updated rationale for a new generation of conservative isolationists and other Republican enthusiasts of a new de'tente with the Soviet Union. If the bipartisan alliance forged by these defeatist ideas should prevail, the decline of American power really would become irreversible -- and so would the decline of the democratic values whose survival still depends on the maintenance of that power and on our willingness to use it.