"Stop it," cries Sen. Daniel P. Moynihan to the Soviet Union, lamenting that Americans have "so little sense of what is connected to what" and declaring that the measure of our distraction in Central America is precisely the measure added by the Soviet hand. Stop supporting the Salvadoran insurgents, Moynihan said to the Russians last Sunday, "or else feed yourselves for the next 10 years, and find a way to pay for a bankrupt Poland, and as many other unpleasant things as we can imagine."

Who has not felt at least some of Moynihan's frustration? Surely the Russian factor has had an important impact in transforming regional turmoil into strategic discomfort in Central America, but nonetheless we have failed to give Moscow sufficient incentive for restraint.

As it happens, President Reagan, even while he underlines the Soviet factor in an effort to build support for his Latin policy, has taken up only half of the Moynihan complaint. "If you go to the source" of the arms Nicaragua sends El Salvador, he said this week, "I think you're talking about the Soviet Union." A week earlier he had charged that the Soviets and Cubans, "operating from a base called Nicaragua," are conducting ''the first real communist aggression on the American mainland."

Presumably not by accident, Reagan has not added to this indictment a promise to act on the basis of it. "They know," he said, meaning that the Russians know what supplies they send to Central America, "and we have communicated to them how we feel about this, and we have also (communicated) to our friends in Cuba . . ." True, the words were uttered while the fleet was under way. They sounded careful and well-considered, as one would hope they would be.

Now Reagan often underplays or ignores the local factors in Central America's torment. But undeniably, I would say, Soviet/Cuban patronage has given El Salvador's home-grown rebellion some part of its staying power and edge and the whole of its hostile strategic potential. Common sense has convinced most observers of that much.

A glance at the wider political scene, however, makes plain that Soviet/Cuban patronage has not been proven in El Salvador in the leading-cause, hard-evidence courtroom sense that a Vietnam-traumatized public has come to demand as a condition of a direct, uninhibited American response. Certainly Reagan has not proven it to the extent--beyond a reasonable doubt --that a public wary of him demands.

Nothing so weakens his case as his failure to still the debilitating argument over whether his inability to present hard evidence of Soviet-Cuban-Nicaraguan arms shipments to El Salvador arises from a necessary protection of intelligence sources, as the administration claims, or from the fact that he doesn't have much of a case. Why won't he tell us?

As a result, he lacks a strong mandate to hold Moscow or Havana directly to account. Any effort to do so will likely heighten the argument over Reagan's leadership that already rages across the board.

I suspect this is some part of why Reagan this week fingered Moscow as "the source" but did not, with Moynihan, drop the other shoe and commit himself to do something specific about it--though his public allusion to past private warnings to Moscow about its supply buildup was suggestive. The allusion was accompanied by assurances that he is "not broadening a war but trying to limit it and trying to bring about a peaceful and political settlement in Central America."

The Reagan administration came to power viewing the world as a seamless geopolitical web; it was brimming with notions of how to counter Soviet trouble- making by applying American counter- pressure in a time, place and manner of American choosing.

Some of the leading possible American responses, however, have since had the ground cut out from under them. To mention two cited by Moynihan, sad experience indicates that our political society has little heart for a Soviet wheat embargo, the tactic of first choice whenever the Soviets do something that outrages us. Nor is there evident a consensus to keep denying the Poles renewed access to Western trade and credits, for the sake of inducing Moscow to ease out of Central America.

Some Americans wonder if we shouldn't simply assert a claim that Central America is in our sphere of influence. But are we then prepared to grant Moscow its sphere --including Poland?--or to halt American support of insurgencies in Afghanistan or Cambodia? Neither the historical record nor the current state of Soviet-American relations suggests Moscow and Washington will soon move that way.

That would seem to leave Reagan with a requirement to resolve the Central American crisis by some combination of actions within the region's own geographical and political bounds. That seems to me the best way to go.