FROM THE GUERRILLAS in El Salvador comes a proposal that addresses the central fact of the military and political stalemate in that unhappy and all-but-forgotten land: that very heavy losses are inflicted by the armed forces' tactics in shelling and bombing areas where people live and by the guerrillas' tactics in conducting sabotage and laying mines. The proposal is that both sides negotiate an end to such tactics in order to ''humanize'' the war -- to confine its military consequences to military forces -- and that this be done in talks separate from the process in which the two sides have tried and failed for some years to come to political terms.

You can ask why either side would use tactics that so hurt the people in whose name it fights. But the guerrillas, it appears, have paid a heavy political price for machine-gunning buses, burning crops, causing perhaps thousands of casualties by mines and the like. Perhaps they also thought to say something public at this time in order to muffle their rejection of Costa Rica's regional peace plan, which is hard on guerrillas challenging governments. In any event, by their proposal to ''humanize'' the war, they have addressed a crying need of their country and put President Jose' Napoleo'n Duarte on the spot.

Salvadoran authorities shy from the new guerrilla proposal. It would be very hard to bargain and to police. It reduces the role of a military accustomed to its prerogatives. It lets the war go on. It does not deal directly with the political conflict. It does deal indirectly, but in a way officials do not favor: one of the guerrillas' ''18 points'' would immunize their political cadres -- evading the elected government's demand that guerrillas simply join the official political process.

Some of these objections are the sort the government would presumably take to a negotiation. Against all of them must be set the appeal of diminishing the brutality and devastation directed against civilians in a country weary of war.

If El Salvador were willing to negotiate on a guerrilla proposal to abandon sabotage, then the question would be sharpened of why Washington sponsors a Nicaraguan guerrilla force wedded to sabotage, which, no less in the one country than in the other, is a tactic designed to make life miserable for civilians. There is, of course, no good answer.