THE ARIAS peace plan leaves El Salvador in a deceptively advantageous place. The plan was designed to deal with Nicaragua, and, being drawn by governments, it favors the Sandinistas: it ends foreign support of local guerrillas and opens the political system only to ''unarmed internal political opposition groups.'' These terms led a wary Nicaraguan President Daniel Ortega to accept a commitment to conciliation and democratic reform. The same terms led an exuberant Salvadoran President Jose Napoleon Duarte to embrace the Arias plan. El Salvador, an imperfect but striving democracy, can live with the Arias goal of democracy far more easily and credibly than Nicaragua can. And in putting an international stamp on delegitimizing guerrillas, the Arias plan handed President Duarte a major boon.

So it appears that El Salvador is in a no-lose situation. If the Salvadoran guerrillas endorse the Arias rules, halt their cruel economic sabotage and their military raids and join the legal political process, the Duarte forces will have a triumph. This result would also give something to those on the left who believe in democracy and reform, not just in destruction and revolution. But if the guerrillas reject the Arias plan -- and they largely have -- they will suffer isolation. Especially will this be so if the Sandinistas make good on their pledge to cut off support for the Salvadoran insurgents, who are a good deal less needful of outside maintenance than the Nicaraguan contras but still take some Sandinista aid.

Let there be no premature celebration for President Duarte, however. For all of the advantages he can extract from the Arias plan, the Salvadoran guerrillas retain an independent war-making capability, a political constituency and a territorial base. A plan that offers them less than what they were trying to get from their earlier unproductive ''dialogue'' with President Duarte is unlikely to stop the war. The formal terms of the Arias plan, drawn with Nicaragua first in mind, need to be adapted to Salvadoran circumstances -- something that lingering feudal elements in El Salvador resist. In short, President Duarte has a promising agreement to work with, but he has his work cut out for him too.