Beneath the Reagan administration's plan -- disputed but advancing -- to take Kuwaiti tankers under American wing, a broader, more political and more positive Persian Gulf policy is struggling to get out. The interesting and ironic thing about it is that an administration described in both friendly and critical rhetoric as determined to assert American power on its own is moving to a policy based increasingly on working through and with the United Nations.

This comes about not simply through the frustration of American initiatives but, in the first instance, through an unusual overlap of short-term Soviet and American interests. The two superpowers are ready to temper their fundamental rivalry for Third World regional influence to a point to achieve their separate ends: to calm the Gulf, to hold or rebuild a relationship with both Iran and Iraq, and in particular to ensure that a rampant fundamentalist Iran does not win the war.

But this is not the only reason the administration goes to the United Nations. In a key aspect of Soviet-American Gulf competition, Moscow suddenly has an advantage. It is in touch with both sides: it arms Iraq and talks with Iran. But Washington is in touch only with Iraq. Before a nervous Washington looms the specter of another ''Tashkent'' -- a unilateral mediation of the Iran-Iraq war similar to Moscow's mediation of the 1965 war between India and Pakistan.

Lone Soviet settlement of a war in an area of traditional Western dominance is as unwelcome to Washington as, well, as lone American mediation of the Arab-Israeli dispute now is to Moscow. Better to fold Moscow into a U.N. approach.

Why, then, does Mikhai Gorbachev not proceed with a second Tashkent? Because the Soviet reach is not all that great and because the Kremlin wants to defuse the current crisis lest the United States use it to achieve ''long-harbored plans'' of building up its military presence in a region on the Soviet doorstep. Moscow's latest proposal to have all non-Gulf states withdraw warships is not serious: they end a small new flier, we end a long Western predominance. But we should not altogether dismiss Soviet strategic anxieties. We are seeking new landing rights, bases, etc.

In any event, it is no sudden conversion to a philosophy of liberal internationalism that finds a conservative administration, one supposedly given to ''global unilateralism,'' knocking at the U.N.'s door in respect to the Gulf. It is hard-headed national interest, and a touch of desperation.

The secretary of state himself is heading to the Security Council next week to cast the American vote for a resolution appealing for a cease-fire and a negotiated end to the Iran-Iraq war. The Kremlin is aboard, as are the three other council members with a veto.

No one expects Iran to buy this resolution. The ayatollahs remain resentful of the council's past one-sided pro-Iraq stance, though the council figures to start making amends by setting up an inquiry into the origins of the war. Tenders of postwar reconstruction aid are also in the cards.

The more modest hopes entertained for the Security Council resolution are that it will help launch the U.N. secretary general into a reasonably early mission to see to putting the resolution into effect. That Javier Perez de Cuellar has worked hard to make himself personally acceptable as an interlocutor in Tehran supports a cautious hope that the Iranians will not give their usual brutal rebuff to a council decision.

The American government would like to be able to threaten Iran with an arms embargo if, as expected, Iran rejects the council call for negotiation and if Perez de Cuellar fails to generate support at least for a lull in the fighting. It seems, however, that an embargo remains a long shot. Too many countries sell arms to Iran or have political hesitations.

All this leaves President Reagan moving toward the reflagging. My sense is that having started it, he should go through with it and meanwhile try to use the new dynamic of joint Soviet-American urgency to switch the focus off the navigation issue, which is a diversion, and to bear down on getting something political going through the U.N. While he's at it, he might make sure that his government, which lags unconscionably on its U.N. dues, pays up in full.