ITS HARD TO EXAGGERATE the extent to which relations between the Soviet Union and the United States rise and fall on readings and misreadings of the other country's domestic political scene. In the last 18 months, for instance, President Carter overloaded the Soviet political circuit, which consists of about a dozen people, with excessive public demands for major internal changes and "deep" cuts in strategic arms. Meanwhile, the Soviet leadership sent waves of distaste and alarm through the American political community by its human-rights violations, African adventures and arms-building programs. More recently, both countries have seemed aware that if they did not act more carefully toward each other, all prospect of substantive improvement in their relations would have to be put off for an indefinite time. Yet a narrowing of the gap sufficient to allow them to work effectively on their most important piece of common business, a strategic arms limitation treaty, has seemed to lie beyond their grasp.

Precisely here lies the potential importance of Sen. Edward Kennedy's visit to Moscow. He went (ostensibly to a health conference) at a moment when the Kremlin was surely grateful to find a prominent American politician, one closely identified with a moderate viewpoint, ready to breach the no-visits line imposed by the administration after the Kremlin's recent human-rights trials. Presumably, the Kremlin also has heard that Mr. Kennedy may some day run for the presidency.

The senator played his part expertly. With a discretion contrasting sharply with the up-front moralistic tones of Jimmy Carter, he asked Leonid Brezhnev about Soviet Jews denied permission to emigrate. As a result, not only the families he asked about, but also a number of families he had not asked about, were assured of release. The Soviet authorities obviously were ready to show that, if their pride and sense of the proprieties are respected, they can behave more flexibly on human rights. Mr. Kennedy's tactful approach let them show it.

On strategic arms the senator seems to have taken the most responsible course a "liberal" in such circumstances could take. Rather than merely profess his desire for detente, he tried to convey a sense of the political difficulties that have been created in Washington for both the negotiation and ratification of SALT by the Soviet Union's own policies. That let Mr. Brezhnev express his concern over the United States' new high-technology weapons programs. He also expressed his belief, shared by his guest, that a failure to consummate a mutually acceptable SALT agreement soon would have harmful results extending considerably beyond strategic arms.

The Carter-Kennedy relationship is the stuff of endless popular fascination. In this instance, the senator, by acting in his own way, was in an excellent position to make the administration's point that progress in SALT hinges in large measure on the Kremlin's readiness to "disarm" the American right by conducting a reasonable policy. That does not mean that the strategic equation itself is of no consequence. It means that the political equation is of great consequence. That is what "linkage" is all about. The evidence of the Kennedy visit is that Moscow is getting the message.