"The Right Way on Contra Aid" {editorial, Dec. 20} is just plain wrong. The Post advocated additional unrestricted aid for the Nicaraguan contras, claiming that more aid would support the Arias peace plan. But this is absurd. The Arias plan specifically called for the United States to stop aiding the contras, and Oscar Arias himself has repeatedly urged President Reagan to suspend aid to the contras while the plan is being implemented.

The Post's call for more aid was no mere academic exercise. Publishing its opinion in the midst of the debate, The Post encouraged Congress to succumb to President Reagan's blackmail and vote more contra aid or face closing down the entire federal government. Failure to do so was equated with squeezing out the contras. But if the contras are truly an indigenous revolutionary force, then surely they can persist until February with no further aid. But if they are, as their critics charge, a Hessian army created by the CIA, then, as The Post apparently fears, they may well drift away from the battlefield when their paychecks end.

We in the United States have long been accused of acting as though we know what is best for Latin America. It's no wonder -- Oscar Arias calls for a temporary halt to contra aid, and The Post demands that it be continued. It's time for all of us, including The Post, to support the Arias peace plan and stop trying to justify a continuation of the war.

WILLIAM R. MILLINGTON Silver Spring