MANAGUA -- A dramatic turn in the debate leading up to this week's vote in Congress on future aid to the contras was the startling revelation that the Sandinista government plans to import large new amounts of Soviet weaponry and to double its armed forces to 600,000 men in the next few years.

Critics of the Marxist regime seized upon the projected military buildup as unimpeachable proof of Managua's expansionist aims. National-security adviser Colin L. Powell declared that the Nicaragua's intention of placing one-fifth of its population under arms posed a "direct threat" to its Central American neighbors.

Yet a different picture emerges in interviews here with Nicaraguans across the political spectrum and western military analysts. They offer compelling evidence that the Sandinistas recognize any military venture beyond their borders could be suicidal. Even if they were fortified by waves of fresh reserves and infusions of new weaponry from a "wish list" submitted to Soviet and Cuban patrons, the Sandinistas seem too overwhelmed by their own troubles to cause mischief abroad.

Besides a near-worthless currency and frequent shortages of food, gas and electricity, the Sandinistas are so obsessed by the fear of an American invasion that they have committed virtually all of their troops and firepower to defensive military tactics. "Right now their primary aim is to survive and stay in power," says a Western military attache.

The primary reason for the military buildup, according to western diplomats and Nicaraguan opposition members, is to ensure future Sandinista political control -- especially if the government extends the kind of democratic freedoms that would be required under the Central American peace plan. The Nicaraguan army, by drafting and indoctrinating youths into the Sandinista revolution, has become one of the party's most powerful tools insustaining its power and support throughout the country.

Besides its perceived political functions, the Sandinista army already has its hands full with the increasingly effective raids launched by the U.S.-backed Nicaraguan rebels. Despite Reagan administraion claims that the Sandinista government is eager to carry out offensive military missions against its neighbors, the Nicaraguan armed forces today are scattered into counterinsurgency units across the country -- trying to cope with 10,000 contras operating inside Nicaraguan territory.

Lt. Col. Manuel Salvatierra, one of the top Sandinista field commanders in the northern war zone, admitted that the contras have improved their military performance in recent weeks but attributed their better showing recently to more sophisticated weaponry and better logistics provided by the United States. Since October, the contras have scored a series of military victories in the countryside, managing to occupy several towns and hold them for several hours. But the Sandinistas reversed some of that momentum by shooting down a contra supply plane last week.

The Sandinista army has evolved from the zealous but scraggly band of 3,000 guerrillas that toppled the Somoza dictatorship in 1979 to a well-disciplined and indoctrinated force of 65,000 regulars today. The army is backed by more than 150,000 part-time reservists and militiamen who receive training in light infantry. The proposed manpower increase would raise the number of regular troops to 80,000 and reserves to 420,000. Yet even if the war against the contras comes to an end soon, the army would shift its primary role to staving off what the Sandinistas regard as an inevitable invasion by the United States.

Under the planned buildup, many of the new weapons that the Sandinistas hope to secure from the Soviet Union will be largely defensive in nature. They are seeking anti-tank guns, anti-aircraft artillery, and minesweepers. But the Soviets have proved unreliable suppliers. Air-defense rockets and rifle ammunition that were promised have not been delivered, and the Sandinistas are still waiting for the MIG-21 fighter planes they want to use to intercept contra resupply planes.

Evidence on the Sandinista military buildup came to light after the defection of Roger Miranda Bengoechea, a former Nicaraguan major and chief aide to Defense Minister Humberto Ortega. Miranda's evidence suggested that the Sandinistas would begin to accelerate their military buildup after 1990, when the Nicaraguan Defense Ministry projects the contra war would finally be eradicated. (The Sandinistas contend that any future military buildup will concentrate on defending their country against the threat of invasion by the United States.)

The Reagan Administration, in turn, has focused on Miranda's claim that the Sandinistas, if confronted by an American onslaught, would seek to "regionalize the conflict." According to Miranda, the Sandinistas had plans to extend the war to Honduras in the north, unleash bombing raids on Costa Rica in the south, and intensify support for leftist rebels still engaged in a civil war in El Salvador. Yet even Sandinista officials say that such military responses would prove fruitless, if not counterproductive.

The Sandinistas, to put it bluntly, lack the firepower to carry out such attacks. Nicaragua's air force does not have the fighter-bomber capability to launch longer-range raids, and its most useful Soviet-provided aircraft is the Mi-24 Hind helicopter gunship. The Hind is useful in attacking guerrilla ground forces, but it wouldn't help much in launching attacks on Honduras, which last year received F5 fighter jets from the United States. Nor would the Sandinistas current contingent of some 150 T54 tanks be able to invade across a hostile terrain of forests and mountains along the Honduran border.

Even today, Nicaragua keeps most of its tanks in Managua, where they occasionally go rumbling through the pock-marked streets of the capitals on maneuvers in anticipation of the day when the Sandinistas fear the U.S. will attack the capital.

William Drozdiak is foreign editor of The Washington Post.