Mary McGrory's column on the Central American peace plan {"Dark Day for the Contra Side," Oct. 15} is a form of wishful thinking that verges on tragicomedy. One need not be an admirer of Assistant Secretary of State Elliott Abrams to know that he is historically correct when he asserts that no Communist country has ever turned democratic. Miss McGrory is no less accurate (adjusting for her tendency to exaggerate) when she says that too few people are listening to Mr. Abrams, and too many want Nicaragua to be an exception to that rule.

This is, sadly, a pattern that has persisted from the Bolshevik Revolution to the present day. Russia, China, Cuba, Vietnam, Zimbabwe and other countries were each supposed to be the exception, and each has found supporters in the West who have warped U.S. policy in accordance with an illusion that denied reality.

None of those countries, nor others like them, has been an exception, of course. Marxist and Fascist dictatorships can no more transform themselves into democracies than sharks can become vegetarians of their own free will. All reasonable people want the Central American peace plan to work, but it takes a peculiar form of ideological self-delusion to believe that it can do so without the prior removal of the Marxist regime in Nicaragua.

Daniel Ortega and his associates understand that and have no intention of leaving. When has a self-proclaimed Marxist leadership ever stepped down voluntarily? The future will look back on Sandinista moderation today as it does on Lenin's New Economic Policy, Mao's "Hundred Flowers" campaign and Hanoi's avowed endorsement of that South Vietnamese "Third Force" it eradicated after 1975 -- seeing one and all as tactical gestures calculated to appeal to the best instincts of the Mary McGrorys and Jim Wrights of the world, whose subsequent misdirection of opinion and policy make them the well-intentioned pallbearers of Western civilization. ALAN NED SABROSKY Carlisle, Pa.