In this season of going for the gold, it matters that the Great Debate of American foreign policy - whether the United States is or is not in decline - has been joined by an adviser of the Dukakis camp, who insists it isn't so.

George Bush is unruffled on this issue. He takes it as an item of political faith that America is No. 1 and should remain No. 1. But his ringing references to the "American century" lack a demonstration to prove the point, or even a sign of awareness that the point does not prove itself. He seems content to make his daily visit to a flag factory.Michael Dukakis, in the part of his message that has been most clearly heard, emphasizes the requirement for the United States to fit its policies to changing world circumstances and to the interests of other countries. This is, of course, an inescapable part of any country's policy. But people are plainly more ready to catch a message of assertiveness than even an intelligent one of accommodation. The latter risks being taken as a national putdown, and Dukakis, for some of his national security views, already has all the headaches he needs on that score.

Dukakis is picking up echoes of John Kennedy by affirmations of "the need to restore respect for American leadership." He said the other day in one of his big security-policy speeches: "I am running for president because I want to lead an America that leads the world, an America that does not settle for second-place or second-best."

Which leads us to the Paul Kennedy question. He is the Yale historian whose thesis of "imperial overstretch" helped spark the continuing debate on whether the new circumstances of the late 20th century, especially the economic circumstances, are making a leading American role a thing of the past-requiring America not so much to lead from strength as to "manage" an inevitable erosion.

Which leads us to the Joseph Nye answer. He is a Harvard political scientist whose article, "Understating U.S. Strength," appears in the new issue of Foreign Policy magazine. Others have contested the economic premises and the historical logic of the Paul Kennedy thesis, but the Nye analysis takes on additional political piquancy by virtue of his status as a Carter State Department veteran who is now a leading policy adviser to the Democratic nominee.

Rebutting Paul Kennedy, Nye says that any decline measured from "the artificial high" of the 1950s is misleading, that the United States is not being challenged by a rising military power, that external commitments are not sapping America's internal strength and that, with certain domestic reforms, we can adapt to the new dimensions of power in the information age.

Although the United States must now navigate among many centers of power and a new measure of international interdependence, Nye says, "Americans should not understate U.S. strength. Misleading historical analogies and false anxieties might prompt Americans to adopt policies of retrenchment that, ironically, could produce the results they are supposed to forestall."

The right strategy, instead, is "to transfer resources from consumption to investment . . . to invest in new technologies, infrastructure, and human resources . . . to invest in defense, aid, and institutions that afford leverage with the international system on the many issues in which the United States is heavily interdependent."

Here Nye detects a fine irony at work: "Reagan's debts," accumulated in the name of restoring American power and prestige, have stirred the belief that the country cannot afford both its international commitments and its domestic investments. Nye says nay: "it is important not to mistake the short-term problems arising from the Reagan period's borrowed prosperity for a symptom of long-term American decline. The latter need not be the case unless Americans react inappropriately to global changes and inflict the wounds upon themselves."