IT WAS ARROGANT of Andrei Gromyko, the Soviet foreign minister, to butt into the Senate SALT debate. His intervention, whatever its intent, was a disaster. There is a double streak as a nation and in the Senate as an institution. Mr. Gromyko affronted both by brassily instructing "those politicians," in English, how to vote. Barry Goldwater's response, "Tell that Gromyko to go to hell," was perhaps predictable. Howard Baker's was serious. A swing figure, he took the Soviet minister's words as "threats" and went on to justify part of his decision to oppose the treaty unless it is amended by declaring, "the American people don't knuckle under."

The Soviets cannot be faulted for wanting the Senate to ratify unaltered the treaty they have committed seven years and their prestige to negotiating. Whether this is the position to which the Soviets will adhere - especially if Mr. Baker's suggestion for mutual cancellation of Soviet heavy missiles and the proposed American MX takes hold - is beside the point: One expects it of them now. But to address senators directly and to lecture them on "objectivity" and "justice" and to warn of "grave consequences" was bound to be inflammatory. As long as Mr. Gromyko was not going to address the substance of senatorial anxieties and objections in an unintrusive and unperemptory way, he should have left SALT-selling to the administration. He only adds to its burden by lobbying tactlessly for the treaty.

It is depressing to see, not for the first time, how poorly the Soviet leadership understands the American political process. (It should also sober those Americans who find one guise or another to intervene directly in Soviet political debates.) Mr. Gromyko has been foreign minister for 22 years and presumably knows the United States about as well as any other Soviet official. He has available a large corps of trained "Americanologists." Yet he evinces an obdurate insensitivity to American ways.

The diplomatic strategies of the two powers have put a premium on showing at least a minimal respect for each other's political sensibilities. Mr. Gromyko's coarseness played directly and negatively into the American debate. He was promoting the very amendments he insists will destroy SALT.