TEGUCIGALPA, HONDURAS -- Concern is growing among Nicaraguan rebels and U.S. officials in Central America that the Sandinista government is making political gains in a campaign to demonstrate compliance with a regional peace plan and weaken rebel morale.

In the view of rebel and U.S. sources, a Sandinista strategy aimed at stalling -- and eventually choking off -- aid to the Nicaraguan rebels is now in full swing and contributing to uncertainty among the insurgents, who are known as contras.

According to this view, the Managua government is attempting to make enough democratic changes to persuade outside observers, notably U.S. congressmen, that it is complying with the Central American peace plan, but without taking steps that would threaten its control over Nicaraguan society.

Although making even limited changes represents some risk for the Sandinistas, whose acceptance of the peace plan reversed a longstanding refusal to negotiate on internal political matters, the potential benefits for Managua are seen as outweighing the drawbacks.

The U.S. and contra sources said the hope of the Sandinistas, who are astute observers of the U.S. political scene, is that Congress will delay consideration of new aid for the contras long enough beyond Sept. 30 -- the expiration date of a current $100 million program -- that another major aid package for fiscal 1988 would be a political impossibility in an election year.

The chain of events is starting to stir fears among some contras and their American backers that contras will perceive further major fighting against the Sandinistas to be in vain. "Nobody wants to be the last guy killed on the battlefield," a diplomat said.

The Central American peace plan, signed Aug. 7 by the presidents of Nicaragua, Honduras, Guatemala, Costa Rica and El Salvador, calls for cease-fires in the region's wars, amnesties, democratic reforms, the cutoff of aid to insurgencies and the removal of rebel military bases in neighboring countries.

Although all of the provisions are to be implemented simultaneously 90 days from signing under the terms of the accord, the Sandinistas have been portraying any new aid to the contras before then as a violation of the agreement. Central American leaders generally have expressed opposition to U.S. consideration of new aid before Nov. 7 on grounds that it would give the Sandinistas an excuse to renege on their commitments.

The maneuvering coincides with a Sandinista propaganda blitz inside the country aimed at getting contras to give up their guerrilla war and accept an existing amnesty. One government poster being widely distributed shows Nicaraguan Cardinal Miguel Obando y Bravo, a leading critic of Sandinista rule, shaking hands with President Daniel Ortega during a recent ceremony to form a National Reconciliation Commission as called for in the peace accord.

As part of an effort to counter the Sandinista campaign, the contras organized a demonstration on Tuesday by about 2,500 Nicaraguan refugees and Honduran supporters at the border crossing of Las Manos. As Honduran soldiers in battle gear kept the crowd behind a chain across the road, loudspeakers blared anti-Sandinista slogans and speeches toward a deserted Nicaraguan border post 100 yards away.

In one message read to the crowd, rebel military commander Enrique Bermudez said the Sandinistas were trying to "undermine the morale of our combatants" through letters and radio messages addressed to contras, their relatives and refugees in Honduras and Costa Rica.

"Ortega signed the accord because he feels harassed by our fighters, and he wants to stop the aid from the American Congress and gain time to consolidate himself in power," the message from Bermudez said. "We must remain firm in our positions. We cannot be ingenuous and believe that the Sandinistas, confessed communists, will become democratic overnight."

Bermudez and other contra military and political leaders reject the Sandinista amnesty as presently constituted, and they publicly deny that the propaganda campaign and the prospect of a new aid cutoff are having much impact on their forces. But U.S. officials in the region and contra sources here suggest that a battle for the hearts and minds of foot soldiers is intensifying, and that contra morale is much more fragile than has been publicly acknowledged.

"This group {the contras} will not stay together if aid dies now," said a U.S. official. "It's a dangerous time for them." He said there was enough aid in the pipeline from the current $100 million package to keep the rebels going "at a reduced level" until November, but that an aid cutoff then loomed larger because "nobody wants to face this issue again before the 1988 election."

"Once we cut off aid to the {contras}, the Sandinistas have won," the U.S. official said. "You can expect them to be very sophisticated in showing that the system is really opening up without doing anything irreversible."

According to a European diplomat, "Everything hinges on the state of contra morale, rather than the state of contra materiel." He said he felt that "contra morale is down, very fragile" and that combat operations have declined significantly from a month ago.

A military spokesman for the contras here, Bosco Matamoros, reported a marked increase in rebel activity in August compared to July, with the monthly number of combat operations in northern Nicaragua rising from 184 to 219, but no overall figures for September were available. A diplomatic source said rebel activities might have dropped off lately because units have reached their infiltration destinations and are "hunkered down" in anticipation of a cease-fire in place.

Meanwhile, according to the European diplomat, "there's absolutely no doubt that the Sandinistas are playing the Democrats {in the U.S. Congress} for all they're worth. They're almost overdoing it." He cited Ortega's decision to release the jailed leaders of a Nicaraguan human rights organization and bar association to Sen. Tom Harkin (D-Iowa), a leading opponent of the contras, six days before their summary 30-day sentence for "disturbing public order" was due to expire.

A few days before, the Sandinistas had refused to allow Republican presidential candidate Sen. Robert J. Dole of Kansas, a supporter of the contras, to see the two men. They were arrested during an opposition demonstration Aug. 15.

Other recent Sandinista measures that are seen by contras as merely "tactical" include permitting the return of two expelled priests, releasing 16 political prisoners and repealing a law that allowed the confiscation of exiles' property.

{The contras, in an initiative of their own, said in Costa Rica that they would release 80 captured Nicaraguans on Friday to show "good will and total acceptance" of the peace plan," The Associated Press reported from San Jose.}

According to Azucena Ferrey, one of six directors of a contra umbrella group, the Nicaraguan Resistance, basic changes to bring about true democracy in Nicaragua must include severing the control of the Sandinista National Liberation Front over the Nicaraguan Army and security apparatus.

While the Sandinista Front claims to be just one of 12 officially recognized political parties, its status as a military organization and its control over society as a whole are viewed by the Nicaraguan internal political opposition as fundamental to a perceived underlying Sandinista aim of developing, in effect, a one-party state.