The Sandinista leadership is watching the heated U.S. debate over further military aid for Nicaraguan rebels with a public air of quiet defiance.

The position reflects increased confidence after a year of military progress against the insurgents and a conclusion that bombastic declarations from Managua and high-profile lobbying in previous Washington debates failed to advance the Sandinista case.

Government officials, following the new line of caution, held back today from commenting on President Reagan's speech of last night, despite his vehement reiteration of U.S. charges against the six-year-old Sandinista government that have been swiftly contested here before.

During a similar debate last spring, Foreign Minister Miguel d'Escoto and other high officials made repeated appearances on U.S. television to denounce administration claims.

After an initial negative vote then, Congress switched and approved $27 million in nonlethal aid to the rebels. Many congressmen said President Daniel Ortega's visit to Moscow immediately after that first vote played a critical role.

In any case, diplomats here said, the episode helped convince Sandinista leaders that appeals to U.S. public opinion were less effective than they had believed.

Sandinista leaders also have expressed the belief that, whatever the outcome of particular steps along the way, the Reagan administration is determined to pursue military pressure against them -- perhaps including direct intervention -- because of an assessment that the Sandinista government represents unacceptable danger to U.S. interests in Central America.

In that light, diplomats and Sandinista officials have said, the fate of one vote on aid to the rebels matters less than the long-term hostility they perceive coming from Washington, and the resolve here to stand up to it.

Vice President Sergio Ramirez gave voice to these sentiments after a meeting Friday with nine visiting U.S. congressmen, declaring: "However the rebels come, we will defeat them."

"We told them clearly," Ramirez said of his talk with the congressmen, "that this debate in Congress to decide on financing the contras is immoral, unjust, illegal and violates the norms of international coexistence as well as the very laws of the United States."

Ortega and d'Escoto, who in the past have taken the lead in enunciating Sandinista policies to the public, traveled to Stockholm last weekend for the funeral of Olof Palme. Even before their departure, officials made it known that the government planned a low-key approach to this week's vote on rebel aid.

In some ways, observers here said, Latin American governments have provided more effective dissuasion than Sandinista propaganda could have hoped to achieve in any case. The leadership here has welcomed recent calls on Washington, urging it to negotiate, from the four original Contadora nations and their South American allies known as the Support Group.

These nations "are trying to avoid" a U.S. intervention, said Interior Minister Tomas Borge in a speech last week to Latin American sociologists. "This is what Mexico, Panama, Colombia, Venezuela, Peru, Argentina, Brazil and Uruguay are trying to avoid. This is the worry of Europe, the anguish of nonaligned countries, the worry of the North American people themselves, who intuit the price in blood they would have to pay if they saw themselves involved in a senseless adventure against the land of Sandino."

Panama, Venezuela, Colombia and Mexico -- known as the Contadora countries since their first meeting on the Panamanian island of Contadora -- have been trying for more than two years to promote a Central American treaty combining continuation of the Sandinista government with security and democratization guarantees for its neighbors. The most recent formulation of their efforts came at a meeting in Caraballeda, Venezuela.

Peru, Argentina, Brazil and Uruguay, the Support Group, have joined to back the effort diplomatically and give it broader regional sweep.

In addition to the changed diplomatic atmosphere, the Sandinista leadership has indicated increasing confidence in the ability of its Popular Sandinista Army to contain the U.S.-sponsored rebel forces. In his speech, for example, Borge declared that the insurgents are "in the process of an irreversible demoralization."

Sandinista leaders have been buoyed by the rarity of attacks against this year's coffee harvesting in the northern mountains. Guerrillas of the Honduras-based Nicaraguan Democratic Force, or FDN, the principal rebel group, in the previous three years had concentrated strikes against coffee farms and trucks carrying the winter harvest to collection centers.

Sandinista officials estimated that more than one-fourth of last year's crop was lost because of rebel harassment, which left 36 coffee pickers dead. This year, they have said, wide deployment of Irregular Warfare Battalions in the coffee region and military convoys to carry the crop to warehouses reduced losses to almost nothing.

FDN leaders have attributed the cutback in their activity to shortages of funds and supplies over the last year. Even the $27 million in U.S. nonlethal aid has been slow to reach the camps of southern Honduras, they have told reporters, because of obstacles to delivery through Tegucigalpa put up by the Honduran Army.