Evidence of Washington's increasingly tough stance on leftist governments and guerrillas in Central America is mounting fast. In swiftly countering Nicaragua's expulsion of three United States diplomats accused of spying, the Reagan administration has adopted what a State Department source called ''a no-nonsense approach.''
Out went 21 Nicaraguan diplomats from the US. And, in a move widely regarded as unprecedented, Nicaragua's six consulates in the US were shut down.
''That is a strong warning to Central America that the United States is no longer going to sit idly by and watch the area fall into leftist hands,'' the Washington analyst added.
Left unanswered, however, is just how far the Reagan administration is prepared to go in implementing its tougher policy. Already, the administration is committing additional advisers to help the beleaguered Salvadorean Army in its struggle with leftist guerrillas. But there can be no mistaking the signs that Washington is embarking on a tougher approach on Central American developments.
This new attitude was underscored in the release June 7 of a letter by President Reagan calling for an end to all foreign aid to leftist guerrillas bent on ousting Central American governments. Although it did not mention Nicaragua directly, analysts say the message focuses on that nation.
Administration spokesmen note that Nicaragua's leftist Sandinista government gets help from a variety of US allies. The spokesmen also contend that Nicaragua , in turn, assists leftist guerrillas in El Salvador.
Washington is saying to these allies, which include many Latin America countries, that the US does not look with favor on any aid to Nicaragua that may in essense be rechanneled to the Salvadorean guerrillas.
The tone of the Reagan letter, sent to Colombian President Belisario Betancur Cuartas, suggests that the US opposes any negotiations with the guerrillas.
Coming on the heels of the administration's ouster of Thomas O. Enders, who had been the State Department's top Latin American policymaker, these developments corroborate the hardening US stance. Enders had favored at least limited negotiation.
This hardening bothers Hemisphere leaders, including President Betancur, who had hoped the US would help to chart paths toward peace in Central America. Without US help, Latin leaders worry that their efforts will be unproductive.
Mr. Reagan's letter, according to a Colombian spokesman, seems to rule out such help. ''It may even spur escalation of warfare in Central America,'' he added.
That view may be extreme, but it is clear the toughening Reagan approach strains US and Nicaraguan relations almost to the breaking point. Expulsion of 2 l Nicaraguan diplomats in exchange for three US diplomats was seen by some as more than tit-for-tat.
And in closing the six Nicaraguan consulates - in New York, San Francisco, Miami, New Orleans, Houston, and Los Angeles - the US took an unprecedented step that will severely limit Nicaragua's activities in the US.
Washington says Nicaragua has used the consulates for espionage activities, but it has refused to amplify on that accusation. Meanwhile, Nicaragua has arrested two opposition leaders, claiming they were in close contact with the ousted US Embassy officials.