Why a small Central American country ignites heated debate within the United States; Soviets heat up war of words on Central America

The ''El Salvador issue,'' remarked one senior Soviet official March 10 with no evident dismay, is ''shifting the focus'' of US public opinion away from the Polish crisis.

Clearly betting on this, and convinced that the United States would not seek an early improvement in superpower relations even without a crisis in Central America, the Soviet Union has sharply escalated its public support for Nicaragua , Cuba, and for ''popular'' forces in El Salvador against the alleged ''aggressive'' designs of Washington.

Whether such support includes indirect military backing for the revolt in El Salvador remains impossible for foreign correspondents here to determine. Moscow officials, and their dependent news media, have routinely shrugged off such suggestions.

But diplomats argue that one thing is clear: To the extent that Cuba and Nicaragua are providing such military support, this would have to be with at least tacit Soviet approval.

The most visible Soviet involvement in the Central American crisis is verbal -- and has been mounting sharply since a period of relative restraint immediately after Ronald Reagan moved into the White House last year.

The campaign reached a new crescendo late March 9, almost precisely as US officials were airing alleged evidence of a new military buildup in Nicaragua (evidence dismissed by the Soviet news agency as ''invented''). Soviet President Leonid Brezhnev, digressing in a banquet address for the visiting President of Finland, declared that the situation in Central America was ''obviously deteriorating,'' and added: ''We resolutely condemn the threats against Cuba and Nicaragua which are . . . heard from Washington.''

''We consider US interference in El Salvador's affairs on the side of the bloody dictatorship of the military junta, which is bent on suppressing the patriotic and freedom-loving forces of that country, to be reckless and extremely dangerous to the cause of peace.''

Mr. Brezhnev also delivered the first official Soviet support for the recent diplomatic initiative of Mexican President Jose Lopez Portillo. In wording seen by diplomats as seeking to head off the impression of a direct Soviet role in the issue, he said that he could ''fully understand'' Cuban and Nicaraguan backing for the initiative. The Soviet media have, in effect, accused Washington of trying to torpedo the Mexican move.

At last year's Communist Party congress here, Mr. Brezhnev omitted all mention of the Salvadoran conflict in a lengthy keynote address, referred only fleetingly to Cuba and Nicaragua, and put greater stress on relations with decidedly nonleftist states like Brazil and Argentina.

Several months earlier, a visiting Nicaraguan delegation had left town with what Western diplomats termed a pledge of only paltry Soviet economic backing.

But since then, Mr. Brezhnev and other officials have gradually played up ties with Nicaragua and, particularly, Cuba. The Soviets have pledged increased economic aid to the Nicaraguans. The Nicaraguan defense minister met with his Soviet counterpart here last November.

On Dec. 7 of last year, the Spanish news agency Efe reported the Soviet ambassador to Nicaragua as saying: ''In case the American government should decide to attack Nicaragua, the USSR, which has supported Nicaragua, is going to support it in its struggle for peace, security, defense, and for its reconstruction.''

The Soviet government newspaper Izvestia, writing on Cuba March 9, declared that the US ''keeps accusing Cuba of 'stimulating terrorism' . . . while openly patronizing the bandits who are getting ready to topple the Cuban government.''

''Solidarity with the (Cuban) island of freedom is growing all over the world. The demand 'hands off Cuba' is sounding louder than ever before.''