With the inauguration of an interim president - Alvaro Magana - El Salvador takes another stride in its pursuit of democratic government under Army domination. The need now is to ensure the progress of economic and social reforms essential to underpin democracy. The minimum was expressed as long as a month ago by a longtime opponent of reform, Roberto d'Aubuisson, whose public stance seemed somewhat tempered after his extreme right-wing party's election success: ''At no time will we take one step backward with regard to the changes that have been taken in our country.''
Mr. d'Aubuisson's attitude is important, because his post as head of the Constituent Assembly is seen by many as more powerful than the national presidency. The assembly, controlled by a right-wing coalition, voted itself extraordinary authority in such matters as writing a constitution, fashioning legislation, vetoing presidential appointments. Just last week it repealed decrees that had granted the executive branch broad leeway in making policy.
Enter President Magana, who has been a banker through many changes of government, maintaining ties with the military but not with any particular political party.He was regarded as sufficiently centrist to respond to the Army's concerns that a right-winger of the d'Aubuisson stripe might affect the military and economic aid flow from the US Congress.
Mr. d'Aubuisson bitterly complained about military pressure to elect Mr. Magana. But he was said not to have anything against Mr. Magana himself, whom he considered a person of competence. Perhaps here there is a basis for getting together and making the so-called ''government of national unity'' more than a phrase.
Part of the ''unity'' compromise was the inclusion of a vice-president from each of the three major parties - notably the Christian Democrats, who have been pretty much left in the cold despite winning a plurality in the election. This, of course, is the party of Mr. Duarte, who was president of El Salvador's military-civilian junta. It is ironic that one price of the elections was the loss of this widely respected leader who was favored by the junta's Washington supporters. But the workings of the system itself must be welcomed as a step forward, as Washington has hailed it. Mr. Magana was not simply named as president of a junta but elected by an elected legislative body to be the President of his country.
Will the Army, which evidently played such a role in his election, keep hands off enough to let democracy work further? The United States can be of help here by following through on congressional requirements that US support be linked to progress on human rights.
Meanwhile it is good to hear that President Magana sees need for at least some kinds of social and economic reforms in a ''sensible'' program.