THE NICARAGUAN contras, whose first reaction to the Central America peace plan was fear of defeat and abandonment, are now interested. Emboldened by recent battlefield successes and by the general alarm over disclosure of Sandinista plans for a huge Soviet-aided military buildup, they are looking forward to winning approval from the two ''juries'' hearing their case. One jury is the Central American presidents, who gather on Jan. 15 in San Jose to assess the progress and prospects of the peace plan. The second is the U.S. Congress, which will be deciding, partly on the basis of the San Jose verdict, whether or how to keep financing the war. Smelling success or faking it, whichever it is, the resistance is toughening its bargaining terms. Its priority is to gain Sandinista acknowledgment that it is a proper partner to a political deal.

Certainly the rebels are entitled to a political role. They are currently in an improved strategic position to claim such a role. The considerable popular support they have mustered belies the charge that they are exclusively a mercenary caste still dominated by elements of the discredited Somoza past.

But to say that contras and other Nicaraguans should be able to challenge the Sandinistas in a political arena is not to say that they and their supporters are relieved of showing judgment and balance in pursuit of this goal. The political opening that the peace plan makes for the region's resistance movements centers on an appeal for reconciliation. The signing governments, most of which face guerrilla challenge, made sure that the opening falls short of an absolute requirement for talks between ins and outs. If this is a defect of the peace plan, it is also a deliberately crafted part of the plan. It cannot be dismissed without sanctioning the Sandinistas' dismissal of other parts that they happen to find burdensome.

If not by the procedures the contras call for, then how does Costa Rica's Oscar Arias, who designed the plan, mean to build the democracy that he rightly says is the region's destiny and its only guarantee of peace? His approach rests on an intent to build trust by mutual compromise and international attentiveness. Too vague? Too unrealistic? Too difficult to coordinate? All of the above. But still perhaps better than any alternative so far in view.