FIVE YEARS of American support for the contras as a military force failed to gain them a political role in Nicaragua, but now American diplomats are trying to use the Arias peace plan to the same end. It's the right end, and it deserves broad Latin support, even though the going in Managua is uphill.

The terms of the peace plan, written by the region's governments, are stacked against guerrillas challenging those governments. The plan ensures a political break only to ''unarmed internal political opposition groups.'' In Nicaragua, these groups tend to be brave but divided and undermanned. The armed Nicaraguan resistance is another story. It includes elements of the country's discredited Somoza past, but it also includes substantial democratic, middle-class and peasant elements, which could become vigorous in open political competition. The peace plan requires the Sandinistas to offer them only individual amnesty, not space for a corporate political role. This is what American diplomacy now seeks to change.

The entering wedge is the issue of a cease-fire. The United States urges the Sandinistas to negotiate this matter directly with the contra leadership. The carrot for the Sandinistas is the promise of a more orderly and reliable cease-fire. But as everyone in the region understands, such a procedure would amount to a first stage of Sandinista political recognition of the resistance. Managua resists, and counters with an offer to arrange local cease-fires with contra field commanders. This might meet minimal standards of a strict reading of the Central America peace plan. Still, a broad and -- we would argue -- true reading requires regional governments to take good-faith steps to bring about genuine political reconciliation. Obviously this is better done when a government negotiates with its major opposition -- always assuming that the opposition plays by fair political rules.

El Salvador's elected president, though he is not absolutely required by the peace plan to do so, is attempting to work out a cease-fire with the leftist guerrilla front struggling against his government. He is making this test of guerrilla good faith in order to exploit the full potential of Central America's current reach for peace. If Jose Napoleon Duarte can act in this bold and statesmanlike way, why can't Daniel Ortega too?