THE CLOSING of La Prensa in Managua 15 months ago constituted a grim tightening of the Sandinista grip, and so it is only fair to hail the reopening of this newspaper as a harbinger of loosening. The Nicaraguan political opposition will again have a voice -- presumably a full-throated and uncensored one, for otherwise the gesture is a farce. No single step the Sandinistas could have taken to meet their early obligations under the Central American peace plan has the same potential importance inside the country and the same resonance outside.

But it remains only a single step. Secretary of State Shultz suggested recently what a reasonable ''scorecard'' might be: return of all expelled priests and not just the best known, release of all the thousands of political prisoners and press freedom extending well beyond ''censored stalwarts'' like La Prensa. The ''basic test,'' he said, is whether Daniel Ortega engages ''the {contra} resistance and the other {internal} Nicaraguan opposition parties in a true dialogue.''

Here is the pinch. The Sandinistas are a Marxist-Leninist party profoundly antagonistic to democracy. It misreads them to say they are not true believers. They have rewritten their country's basic law, for instance, to subordinate the nation's army and security forces to their party alone. This is how President Ortega comes to have the authority to ''authorize,'' as the announcement said, reopening La Prensa. What he can give he can take away. Reopening La Prensa seems to be the sort of small test he is ready to face not to bring democracy but to avoid democracy. Using what influence is available to insist that the Sandinistas move on from small tests to large ones is the challenge before the Latin democracies now.

The Reagan administration is convinced that the clear certainty of resumed contra aid would be the most useful form such influence could currently take. Their history and politics, however, lead most Central Americans to see that as an unacceptable form of American intervention. They did not embrace a peace plan thinking there was the stuff of a second democratic Costa Rica in Nicaragua. They did so believing, on the basis of six years' experience, that they preferred the rigors of peace to the rigors of continued contra war. This was the moment, they figured, to induce the Sandinistas to buy off the contras' attack with progress toward pluralism. The reopening of La Prensa indicates the progress made -- and the progress still to be made -- in this risky passage.