THE SANDINISTAS in Nicaragua have gone back to the Cuban-type police-state rules they suspended last year during the election period, when a hint of openness to pluralism was deemed convenient. Even in the interval, arbitrariness and repression were common. Now the totalitarian core of the Sandinista philosophy is out in plain view.

For this turn the Sandinistas blame the contras and their American supporters, claiming that the elements they are clamping down on serve enemies of the revolution. The Nicaraguan regime's credibility, however, is low. Facing a crisis, the regime could have moved to reconcile the nation. Instead, the Sandinistas have increasingly alienated Nicaraguans living under their rule and refused to countenance any dialogue with those Nicaraguans who -- many of them anyway -- were driven to take up arms by the Sandinistas' repudiation of their own revolution's democratic promise.

In a perverse way, the crackdown seems to be a more reliable index of popular feelings than the tilted elections the government ran last November. Certainly, it represents an authoritative government view of its standing among the people.

The first result of the sweeping new restrictions is bound to be to make life more miserable for Nicaraguans. Another likely result of closing out what opportunities existed for peaceful and legal opposition will be to channel resistance into armed struggle. The Reagan administration had been contemplating how best to obtain the next slice of aid to the contras. It has now been given an enormously useful ready-made case to take to Congress. That is why apologists for the Sandinistas are anxiously appealing to Managua to change its mind.

From this point of view, however, a large cloud remains on the horizon. Reliable reports persist that leading democrats in the Nicaraguan opposition are distressed by the place in its ranks of followers of the former dictator, Anastasio Somoza, and by the human rights abuses still being reported. The Reagan administration's efforts to clean up the contras evidently have a good way to go.

The unhappy truth of Nicaragua is that each side has an investment in the other's bad behavior. The Sandinista crackdown plays into the hands of the contras, the contras' excesses into the hands of the Sandinistas. Both ways, the Nicaraguan people are the losers. A reconciliation of all Nicaraguans prepared to live with each other remains the only decent way out.