There is an odd symmetry to the climaxes now approaching in Afghanistan and Nicaragua, where military struggles appear to be yielding simultaneously to a ragged but undeniably political phase.

In the most hard-nosed and cynical of the ways that have been mentioned (though not officially discussed) to treat these two regional inflammations, Moscow and Washington would have enforced respective spheres of influence, Moscow imposing control on the Afghans and Washington on the Nicaraguans. But no sign is visible of this sort of a great-power deal.

Quite the contrary: Each great power asserts a special security privilege in its back yard, and the United States offers a historical pedigree for its privilege -- the Monroe Doctrine. But each power may be approaching a major humiliation in its traditional preserve. In Afghanistan the resistance looks to be forcing out Soviet aggressors and perhaps also the regime and party they installed, and in Nicaragua the regime appears to be on the way to forcing out the resistance and to be dashing the hopes of the American president who put the resistance in the field.

Mikhail Gorbachev is arguably better off for having made a deliberate choice to cut Soviet losses. We cannot know how this leaves him positioned in Kremlin politics. Internationally, however, the decision entitles him to ask the Kabul regime and the Pakistani patrons of the resistance to do a certain amount of furniture arranging before the Red Army goes home, and it improves his standing to expect the United States to do its part in avoiding a replay of the American evacuation of Saigon.

By contrast, Ronald Reagan is having his losses inflicted by his political opposition, which understands the value of combining military pressure and a diplomatic outlet in Afghanistan but rejects the same combination in Nicaragua. This leaves the president embarrassed and angry and more in a mood to try to reverse the congressional verdict -- though that goal is a long shot -- than to join his Democratic tormentors in making the best of the situation they have wrought.

Both in Afghanistan and in Nicaragua, nonetheless, there appear to be limits to great-power losses. The new regime that we hope to welcome in Kabul will not be one that puts Moscow at a strategic disadvantage: Washington has pledged to support a neutral, ''Finlandized'' Afghanistan. At this point the United States might be delighted to be assured that Nicaragua was going to be similarly ''Finlandized,'' rather than communized. Still, even if the worst happens in Managua, the United States would retain the power of its own and would have the deference from others to care for American strategic interests in the region.

This comes about not through any explicit Soviet-American agreement on ''rules of the road'' for regulating great-power competition in the Third World. Efforts to draw up such guidelines have been under a cloud since the failed Nixon-Brezhnev initiative of the '70s. Rather, a mutual regard for the other's vital strategic interests has been built into the reflexes of the international system at least since the Cuban missile crisis.

In Afghanistan, the resistance is a political fact, and the United States' basic role in supporting it has given us the leverage to get into high-level discussions with Moscow on the future of the country. Moscow, however grudgingly, accepts us as an interlocutor and tries to make the most of it -- by bargaining for an end to the flow of American arms, for instance.

Since the Soviet-supported regime in Managua is also a political fact, would it not make sense for the United States, also however grudgingly, to accept Moscow as an interlocutor there? To discuss, for instance, Soviet arms supplies? To enter parallel discussions with Cuba on curbing the Sandinistas' revolutionary reach?

The Reagan administration believes that to enter such consultations after Congress has removed the principal card in its negotiating hand would convey American acceptance that the Sandinistas are in power to stay -- a second breach of the Monroe Doctrine, after Cuba, and one from which this administration recoils. In fact, there will be plenty of time to consult Moscow and Havana after the Sandinistas, whose professed alibi for monopoly rule (the contra threat) is evaporating, show they are their people's choice.