IT IS A DAY in Nicaragua. On one side is the opposition, which has set today as the deadline for the Somoza family to leave the country; otherwise, the opposition warns, it will prepare to resume all-out war. On the other side is President Somoza, who has countered with a proposal for a plebiscite of all political parties; Mr. Somoza would then set up a government, remaining as president. Since the first round of their struggle in September, both sides have heavily rearmed. All guns are on a hair trigger.

But a second round can still be averted, or so it is possible to hope. The answer lies in the American-led mediation effort. Notwithstanding the opposition's suspicions, the United States, we believe, entirely supports its determination to end the Somoza dynasty. The only difference, a critical one, is that the United States would prefer to see it end by a popular democratic process - in a plebiscite in which President Somoza would agree to leave if he lost. This is very different from President Somoza's plebiscite, which would submit to popular judgement not his rule but merely the arrangement of the parties under him. The Carter administration understands precisely how phony that is.

One can understand why the opposition feels angered and insulted by the thought of having to enter an electoral contest against a man who, it correctly thinks, has forfeited any legitimate claim to hold power. The unanimous new report of the Human Rights Commission of the Organization of American States, describing atrocities the Somoza National Guard committed against unarmed civilians in September, provides persuasive and authoritative proof to outsiders of what the opposition has been sure of all along. Yet the opposition surely is confident of the support it could hope to win in an American-style plebiscite. It should swallow its outrage and follow a path that promises to remove President Somoza - and to do so in a way that spares Nicaragua further violence and that assured that the succession to him will have what the Somoza dynasty has never had, the legitimacy of a popular mandate.

If the opposition accepted an American-style plebiscite and the Somoza dictatorship did not, the administration would have no choice but to withdraw the vestigial support it has left in place during the mediation period. Mr. Somoza's foes would then go at him in their fashion. Part of the Somoza strategy is to hope Washington will fear that a free-for-all will lead to "second Cuba." That is blackmail. Anastasio Somoza is a bloody dictator and he cannot be allowed to hide behind American apprehensions about communism.