Under siege, Sandinistas grapple with rebels, US

State Department analysts expect that Nicaragua's expulsion of three Americans will be followed by a crackdown on opponents of the Sandinista regime. Spokesmen for the State Department and the US Embassy in Managua have vehemently denied Nicaraguan charges that the three American diplomats were plotting against the Nicaraguan government. A State Department analyst said that the charges were likely to be used as a pretext by the Marxist-led government to tighten the screws on opponents of the regime in political parties, labor unions , and the Roman Catholic Church.

As seen from here, the ouster of the three diplomats was as much a message to Nicaraguans as a message to the United States. The message would go something like this: ''We're in a state of emergency. The Americans are trying to overthrow our revolutionary government. We can't tolerate dissent at a time like this. You had better think twice before causing us any trouble.''

Popular tribunals aimed at bringing American-supported ''counterrevolutionaries'' to justice were supposed to open in Nicaragua this month. American officials say that these could lead to a degradation of the judicial system in Nicaragua and abuses of power.

The most serious charge leveled by the Nicaraguans against the three ousted American Embassy officials in Managua was that they were involved in a ''spy network,'' which had as an objective the assassination of Nicaragua's foreign minister, Miguel D'Escoto Brockman. At a news conference in Managua Monday, Lenin Cerna, head of the Nicaraguan Interior Ministry's Department of State Security, displayed booklets allegedly designed to translate codes, note paper that dissolved in water, and a bottle of liquor, or wine, which Mr. Cerna said contained poison to be given to Foreign Minister D'Escoto.

Marlena Moncada, identified as a Nicaraguan consular official, told reporters that a CIA agent had tried to recruit her last year when she worked at the Nicaraguan Embassy in Honduras. She informed Nicaraguan authorities, she said, and then went to work on their behalf as a double agent. Miss Moncada said that after she was transferred to the Foreign Ministry in Managua, the Americans instructed her to give the adulterated liquor to Mr. D'Escoto.

State Department officials scoffed at this allegation. For one thing, they note that presidential executive order guidelines to the US intelligence agencies prohibit assassinations. But they also argue that if they wanted to remove a Sandinista official, D'Escoto would be one of the last ones to be on anybody's ''hit list.'' One State Department official commented: ''D'Escoto's got no power. He's an ineffective front man.''

Cerna said that Linda Pfeifel, a US Embassy political affairs officer, had visited anti-Sandinist political party members and trade unionists in Managua and charged that she pressed them to continue antigovernment activities. But State Department officials said that such visits were part of routine diplomatic work and that they were made openly.

Nicaraguan officials identified two Nicaraguans who were implicated in the alleged American plots. One of them, Mario Castillo, described as the head of a youth group of the Democratic Conservative Party, was reported to have been arrested. The officials indicated that still others belonging to what was described as a network of CIA ''collaborators'' might be arrested.

Some political parties and labor unions are already under considerable pressure in Nicaragua. According to an official in a regional labor organization who asked not to be identified, the Sandinistas now hold some 15 workers from the Nicaraguan Workers Central (CTN) in prison in violation of International Labor Organization agreements. The official said that Nicaraguan workers no longer have either the right to decide freely when and where they will meet or the right to negotiate collective salary contracts with management. What was most frustrating about this, the official said, was that CTN workers suffered significant losses in the battle to overthrow the late President, Anastasio Somoza Debayle.

The Sandinistas now expect, meanwhile, that some officials in the Nicaraguan Embassy in Washington may be expelled by the United States. State Department spokesmen say that countermeasures are being considered, including an expulsion that would amount to more than just a tit-for-tat ouster of three Nicaraguan diplomats.

The Reagan White House and the Sandinista regime now have what amounts to ''devil'' images of each other which will make it difficult to prevent a further deterioration in relations beyond the mutual expulsion of diplomats.

On the Sandinista side, President Reagan is portrayed not just as an enemy of Nicaragua, but as the enemy of all mankind. A few weeks ago, a photograph of Reagan appearing in a pro-government Nicaraguan newspaper was touched up to make Reagan have pointed teeth and fingers and other angular, devil-like features. Nicaraguan textbooks are being prepared which give history an increasingly Marxist focus, with the US presented as a nation that has exploited Nicaragua and repeatedly intervened in its affairs.

The Reagan administration, for its part, has described Nicaragua in terms that some diplomats find to be too harsh. They point out that the Sandinistas have made progress in the fields of health, literacy, and land reform, and that the regime has clearly tried to help the poorest people in the society.