WE PUBLISH today a letter from the Nicaraguan ambassador, Carlos Tunnermann, taking issue with an article in this newspaper on May 6 by Horacio Ruiz, managing editor of La Prensa in Managua. The ambassador does not challenge any factual aspect of Mr. Ruiz's account of the ways in which his government censors La Prensa, which is the sole residual opposition voice in Nicaragua. Rather, he goes over to the offensive, citing the national security requirements of a country at war and linking Mr. Ruiz to the armed opposition to the Sandinistas promoted by the United States. A routine political response, you might say, except that there are at least two things about it that are not routine.

The first centers on Ambassador Tunnermann's linking of La Prensa to the American private organization called Prodemca -- for Friends of the democratic Center in Central America. Prodemca has helped finance the embattled La Prensa, using federal funds channeled through the National Endowment for Democracy, which was set up by the Reagan administration in 1983 for such purposes. But Prodemca, using other funds, also has lobbied for military assistance to the Nicaraguan contras. Prodemca's funding of La Prensa came to a halt in March when the Endowment belatedly realized that Prodemca's indiscriminate mixing of funding and lobbying would be used by the Sandinistas to discredit the newspaper. It is still being so used: note Ambassador Tunnermann's suggestion that Prodemca's goal, "which is the violent overthrow of our government, is also the goal of La Prensa." Perhaps some of these organizations in Washington should be renamed: the National Endowment for Embarrassing Democracy; Unhelpful Friends of the Democratic Center in Central America.

Then there is the irony of it all. Ambassador Tunnermann's government censors La Prensa brutally and, as Horacio Ruiz's article made clear, extends censorship far beyond matters of military and state security. The ambassador's government refuses to let La Prensa voice its complaints of censorship in its own pages. But when a La Prensa editor voices them in an American newspaper, the ambassador hastens to avail himself of the privileges of a free press and to make a response. It is grotesque to have a Nicaraguan discussion, one that would not exist in the first place except for Sandinista censorship, forced into the pages of a foreign newspaper. When will Ambassador Tunnermann's regime allow Nicaraguans to read all about it?