GIVE THE Sandinistas credit for responding to the tough challenge posed to them by Costa Rican President Oscar Arias, who mobilized almost the whole hemisphere behind a demand that they talk to their despised foes, the contras. Daniel Ortega, the Nicaraguan leader, went part way, returning from Moscow in time to meet a key Nov. 5 deadline of the Central America peace plan and to agree to negotiate a cease-fire with the contras through an intermediary. The contras at once accepted the Ortega offer, taking it as a step toward political recognition. The Sandinistas can be expected to resist this reading, but a process has begun.

In the various concessions he made up to and on Nov. 5, President Ortega confounded many Latin and American skeptics. Thus has he reaped a tactical advantage, which he put to instant use by going over to the diplomatic offensive and laying down Nicaragua's preferred terms for the next phase of the Arias plan. Nicaragua will lift its state of emergency and release all but the old Somocista prisoners under an amnesty, he said, if the United States and Honduras cut off the contras. He assigns determination of American and Honduran compliance to the Arias verification commission. It is made up of 13 Latin nations plus the (Latin) secretaries general of the Organization of American States and the United Nations. It is due to weigh in on Dec. 7.

The Ortega response cuts directly across the American strategy to keep the contra force in being as an enforcer and bargaining chip until Nicaragua's passage to democracy is deemed irreversible -- and to leave this determination of irreversibility to Washington. It cuts immediately across the Reagan administration's plan to induce Congress to vote interim funding for the contras this month, pending a request for large new funding in January.

President Ortega is being clever, but he is also acting within the context of a peace plan that the Latins see as the region's last best hope and that the United States has formally if reluctantly committed itself to support. The Ortega gambit does not ensure that peace and democracy are coming to Central America, but it should stir the American government to stop standing on the sidelines and waiting for the plan to founder and, instead, to jump in and try to do what it can to make the plan work for American ends.