The occupational hazard of revolutions is to become mirror images of the tyrannies they overthrow. Freedom-loving Nicaraguans are trying to stem the post-revolutionary extremism which is taking their country in this direction. They are dismayed at United States policies that they see as giving the extremists an excuse for repressive measures. By this reasoning, hard-line leftists are bolstered as Washington continues to avoid diplomacy in favor of supporting armed action by remnants of the old Somoza dictatorship's national guard and other opponents of the Managua regime.
For a time the US appeared to be opening the diplomatic door a crack. It would be wise to pursue this line and renounce any overt or covert backing of force against Nicaragua's Sandinista government. Even the most open Nicaraguan critics of the regime's excesses see US intervention as counterproductive.
But if Washington would be repaid by listening to these critics, so would the regime that has become a magnet for Washington's anticommunist efforts. The latest Nicaraguan to speak out is Francisco Fiallos Navarro, who has been ousted as ambassador to the United States for doing so. He was preceded by exile Eden Pastora Gomez, the redoubtable Commander Zero of the revolution against Somoza, who last spring denounced the radicalism of former fellow-Sandinistas seeking to ''hold on to power at all cost.''
Mr. Fiallos recognizes the Nicaraguan revolution as irreversible but warns against its taking the country from a rightist dictatorship to a leftist dictatorship. He calls for political pluralism, free elections, a lifting of censorship. By returning to such stated ideals of the revolution, its leaders would not only serve their own people but give the United States an offer it could hardly refuse. The Reagan administration has long spoken of such reforms as conditions for restoring relations.
One misapprehension to dispel is the equating of Nicaragua with Cuba as a domino certain to topple toward abject communism and Muscovite serfdom. So far, for example, foreign and many other private businesses carry on in Nicaragua; at this stage after the Cuban revolution almost all private enterprises had been expropriated.
Nicaragua is not down the drain. It can be saved from that future so long as there are Nicaraguans from its broad-based revolution willing to stand up to the self-seeking exploiters of it - and outside powers such as the US willing to respond to their heartfelt pleas.