THE FIRST American response to the Nobel Peace Prize awarded Costa Rican President Oscar Arias was inadequate. Far from praising him for unlocking a door it had been unable to open on its own for nearly seven years, the American government reacted as though the award would only complicate its life in Central America. It took several hours for the administration to compose a response gracious enough to match this great honor to a man and his country. Even then the impression was left that Costa Rica has a lightheaded policy for a suspect ''peace'' while the United States pursues a separate, serious policy to remove the Sandinistas from power.

It is true that some Latin Americans tend to gravitate toward the part of the Arias plan that calls for a halt to the fighting; this can be done easily -- wrongly -- just by ending contra aid. The United States emphasizes the part that compels compensating democratization and reconciliation; indeed, it demands progress to a degree that would ensure the effective ouster of the Sandinistas -- a goal the contras have failed to achieve on the battlefield.

Yet it is very unfair to President Arias to suggest that he has a lesser interest in democracy and reconciliation than does President Reagan. He sees these not just as values in themselves but also as the ultimate enforcers of an agreement that Central America, after all, must live with.

In fact, an idea is stirring that could put the United States and Costa Rica in useful and necessary diplomatic harness: to press Nicaragua to act in the spirit of the Arias plan by opening up a political role not just to the weak legal internal opposition parties but also to the guerrillas; El Salvador and Guatemala are already setting an example. The administration promotes the idea openly, Mr. Arias more quietly. The Sandinistas hang back, but in a change that may make the idea easier to sell in Managua, the administration has stopped making the demand that Nicaragua move up its elections of 1990.

At this point the harangues on the aid issue do not help the administration achieve any of its goals. The talk, we think, does not change the political odds on aid. It merely hardens divisions and distracts attention from the central point, which cannot be stressed often enough: the Sandinistas' obligation to respect the promises of democracy and reconciliation they made to their people and neighbors.