AS THE ARMY'S military effort has flagged in El Salvador, the Reagan administration has come under increasing pressure to encourage talks between the government and the guerrillas. It is responding in a measure not yet fully registered by the American and international publics.

The administration now publicly supports "negotiations within the framework of [Salvadoran] democratic institutions." The United States and other Latins, the State Department says, "should be able to assist the Salvadoran government to provide the guarantees of personal security, of access to media for campaigning, of a fair count, of respect for the results of the votes cast which all participants are entitled to expect." With the approval of the Salvadoran right-wing leader previously known for urging the slaughter of all "subversives," the country's new "peace commission" has been directed to sound them out about coming in.

So, many people will say, what is wrong with that? Is the evolving American position not reasonable and fair? Would it not be acceptable to guerrillas who were sincerely interested in ending the war and in moving El Salvador's struggle to the political arena, rather than interested simply in taking full power?

In many other circumstances, the administration's position would be good enough. There is, however, a special difficulty in El Salvador. It lies in the pervasive mistrust built up as a result of the authorities' massive past abuses both of the electoral process and of the confidence of the left. The administration has come a good way from telling the guerrillas to shed their arms and line up at government polling stations. It still has a good way to go to establish conditions in which conscientious people could insist that the left take part.

The left itself, however, has even further to go. Let us leave aside for the moment the question of whether the men who speak in the name of the guerrillas actually speak for them. Even so, their words have a disturbing sweep. They reject government-sponsored elections in favor of a usually undefined "unconditional dialogue." But first, they say, Washington should cut off Salvador's military supplies. They demand, before elections, a "comprehensive political settlement" that includes overhauling the armed forces, initiating "socioeconomic reform" and altering Salvador's "international position."

In short, the Salvadoran left asks for something extravagant and unreasonable: the moon. It needs to start moderating its demands. Too many critics of the administration allude to talks as though they were magic. People who urge talks cannot shrug off their responsibility to help the guerrillas as well as the government to come to a reasonable view of what a fair process and agenda might be.