THE PRESSURE on the Reagan administration to loosen up its policy in El Salvador is starting to work. Formerly, it characterized the little Central American land as the single most important front where the whole forward thrust of international communism had to be met. Now it says, in the words of the latest States Department presentation, "just as the conflict was Salvadoran in its origins, so its ultimate resolution must be Salvadoran." Many people, including Americans and foreigners entirely sympathetic to Mr. Reagan's purposes, had thought he was vastly exaggerating the stakes at play in El Salvador. It seems he is coming to agree.

The administration has also recast the forcus of its policy within the country. From pointing toward a military victory, it now suggests the solution must be a political one incorporating "a genuinely pluralistic approach." In this spirit, officals are emphasizing a commitment to an electoral process open to "all parties that renounce violence," and efforts have begun to split the opposition and to draw from it those democratic elements prepared to take their chances at the polls.This, too, is responsive to a considerable number of critics, who felt the administration was going too heavy on the guns.

But the administration needs to keep going. The key question is whether the elements in and aroung the Salvadoran security forces -- these forces, keep in mind, are not controlled by the junta's civilian leadership -- could meet the administration's own test for participation in the political process. That process is reserved for "parties that renounce violence." The security forces fight guerrillas; in that struggle, evidently, neither side is anywhere near ready either to achieve victory or to concede defeat -- the formula for a wasting war. But the security forces are also a law unto themselves, and they are the arm of the oligarchs, and in those roles, still, they wreak violence upon the common people. In a country where the army has block free and fair elections for 50 years, elections held under the auspices of these security forces would mean little.

Mr. Reagan does not want to "lose" El Salvador, or to lose in El Salvador. Nothing valuable and enduring can be gained, however, if the result he pursues is not consistent with what the people would choose if they had the choice. It is evident now that, whatever its dimensions, the foreign input on the guerrilla side is not going to make the difference. One critical part of the American input on the government side is to help bring the security forces under control.