THE SUMMIT in Venice is being described as a disappointment for the United States, which wanted a more demonstrative endorsement of its Gulf policy. But leave aside that it was imprudent for the administration to publicize its hopes in this matter in advance. It was even less prudent not to anticipate that the allies would be sobered by the stunning secret turn American policy toward Iran and terrorism had taken during the last few years. For the alliesnow to embrace Mr. Reagan's policies as though he had not raised the deepest doubts about his credibility was never in the cards. Bungles cost.

But what about the allies? Are they as detached as depicted in some quarters? The issue is relevant to the effectiveness of American policy and to the support for it likely to come from Congress, which tends to view allied cooperation as the standard by which it should measure out its own support for the Reagan initiative. The evidence available at Venice was not cheering. But the evidence available in the Gulf is different. The United States is not alone there, as the more alarmist congressional sentiment suggests. The British and French have their warships there too. The American government, to keep itself from looking isolated and reactive, exaggerates the extent of allied cooperation. But some in Congress, putting a strategic gloss on jitters, minimize it.

Besides, the Russians are now in the Gulf. This cuts several ways. The Red Navy has made a historic leap into a region from which it has been the Western strategic purpose for a century or more to exclude a Russian/Soviet presence. This leap created an absolute requirement for any would-be great power to offset the new Soviet presence -- not to be careless, but to act. Instinctively and intellectually, the allies understood this large requirement; that is why, nervous as they may be about Mr. Reagan, they accept his reflagging. Some in Congress have not yet understood, and focus less on the rationale of reflagging than on its risks.

Not out of any love for us, but in pursuit of its own interests, the Kremlin, in moving its fleet into the Gulf, is reinforcing goals held by the West: free navigation, containment of Iran, a negotiated end to the war. The administration is unaccustomed to finding itself with even so scanty and partial a convergence of interests with Moscow as this. But the tactical uses of this convergence are there to be applied. It makes the American intervention marginally safer, and gives Russians and Americans incentive to see the other succeed. The administration, which needs to build international and domestic support for a sound policy, would profit from making the point.