THE POLITBURO is not content simply to berate Jimmy Carter and to object to his reponses to the Soviet invasion of Afghanistan. It predicts the new steps will fail, terming them "a hopeless under taking that will flop." The taunt measures the president's challenge. Plainly, the Kremlin thinks, or at least claims, that Mr. Carter lacks both the diplomatic and political clout to carry off his policy. If he now fails to make a good showing, severe damage will have been done to the national interest -- not to speak of the damage to his reelection campaign.

How well has the Kremlin read the international scene? Keep in mind that the United Nations never condemned the United States on Vietnam, or forced Washington to wield a veto on that issue. But dozens of nations are now assailing the Soviet occupation of Afghanistan. Among those angry or alarmed enough to stand up, there is, to be sure, some nervousness: they don't all want to name Moscow as the aggressor. Among American allies, you will not be surprised to hear, France is finding highfalutin reasons to go its own narrow way. Others in the Third World are similarly hedging: India, for one, calls for Mooscow's withdrawal "at the earliest possible time." Nonetheless, a rare condemnation was delivered yesterday, precipitating the expected Soviet veto; the issue is almost certain to come before the General Assembly.

Many of Afghanistan's fellow Islamic states share the common outrage. Governments like that of Jamaica, no American cat's-paw, are coming on strong. One sign of the swing is Cuba's decision to beat a discreet retreat in its battle for a Security Council seat. (In referring to this battle the other day, we inadvertently said "Chile" when we meant "Cuba" -- and apologize for the error.)

As for the Kremlin's reading of the American political scene, Moscow evidently has been influenced by its own Marxist contempt for capitalist societies and perhaps also by the common conservative lament that Amererican reporters in Moscow that the American public is hooked on the benefits of detente, especially trade, and does not want to lose them.

On this, of course, the jury is still out. We offer, however, a preliminary observation, Mr. Carter's Republican adversaries -- interestingly, especially those who fault him for not being tough -- seemed more eager to criticize the grain embargo over the weekend than did the first wave of farmer opinion. By showing how he means to distribute the farmers' burden fairly among the population at large -- a political and administrative task of no small order -- Mr. Carter can substantially aid his own cause. To the extent that Soviet foreign policy has been based on certain assumptions about the American political process, some surprises could be in store.