MULUKUKU, NICARAGUA -- Nicaraguan soldier Carlos Guzman takes his nightly rest these days on a bed of mud covered with bug-ridden leaves. He dozes to the buzz of mosquitoes swarming around his ears.

With only a sheet of plastic overhead to shield against a tropical drenching, Guzman goes to sleep clammy and wakes up soaking wet. Day after day his jungle boots ooze like sponges as he hikes.

Nowhere is Nicaragua's rainy season, which began this year in July, more gray and unrelenting than in the steamy central forests where infantryman Guzman and his battalion are stalking antigovernment, U.S.-backed rebels, known as contras.

The U.S. Congress is expected to vote in the fall on whether to renew aid to the contras, which came to $100 million in the past year. But judging from the sodden conditions around Mulukuku, a Sandinista Army staging base in remote, contra-infiltrated Zelaya province, the timing could not be worse for the contras to display any fancy fighting before the vote.

Moreover, during a reporter's two-day visit, 20-year-old Guzman and the boyish troops of the Sandinista Army's Rufo Marin counterinsurgency battalion seemed cheerful and confident despite the dampness, their capabilities hardly taxed or tested yet by a contra offensive that began last December.

The battalion is meant to be a cog in a well-oiled Sandinista military machine that controls contra movements in this region while also making true Marxist believers of its recruits.

The Rufo Marin, with about 600 men, is one of 12 antiguerrilla battalions, known by the Spanish abbreviation BLI, in the 65,000-troop Sandinista regular Army. The battalions, created in 1983, have no fixed garrison. They can be dispatched to any corner of Nicaragua in response to contra movements.

"Building the BLIs was one of our Army's smartest moves," asserted Defense Minister Gen. Humberto Ortega in an interview in a recent issue of the Sandinista Army magazine. "They have more political cohesion than the enemy forces . . . and they give us great freedom of movement," he said.

The Rufo Marin set up its headquarters in a Soviet-made tent on a bee-infested knoll in Mulukuku in late May. Last week, four of five companies were on the trail of about 200 guerrillas, believed to belong to a contra force named Santiago Meza, in thick forests six days' march to the northwest. Two Soviet-made Mi17 helicopters that were to fly in supplies and two reporters could not make the trip last week due to rainstorms.

The other company, Guzman's, was patrolling along a dirt road leading to Mulukuku, where a convoy of government flatbed trucks with heavy construction materials was scheduled to pass.

The soldiers spoke with awe of the terrain, which Nicaraguans call la montana, where the main companies -- and the contras -- were operating. It combines the knotted undergrowth of a triple-canopy jungle with steep hillsides and jagged precipices.

"Horrible," said Lt. Carlos Vargas, 26, the acting battalion commander. "Snakes, swamp, darkness -- downright gross."

But the operations officer, Lt. Jairo Rodriguez, 26, was in daily radio contact with his companies, and he said they were not complaining.

They had enough Czech and Bulgarian C-rations. Each company had a medic carrying supplies of chloroquine to fend off malaria, creams to kill foot fungi and tablets to purge parasites, according to battalion physician Cesar del Valle, 23.

They also had grease and ammunition for their Soviet-designed AK47 rifles, light machine guns and AGS17 grenade launchers. Most companies were carrying Soviet-built SA7 antiaircraft rockets in case a CIA-run contra resupply flight should pass within range overhead.

The battalion's relatively complete stocks contrasted starkly with civilian Nicaragua, where shortages of food and medicine are chronic.

The Rufo Marin has no foreign advisers. Two Cuban officers trained the first Rufo Marin contingent in 1983, officers said. But the Cubans did not prepare the men for the trials of la montana. Later draftees were trained by Nicaraguans who knew firsthand what they were in for.

Sandinista officers acknowledged that intensive training last year of the contras in the United States and at base camps in Honduras seems to have improved the rebels' battle skills.

"Their maneuvers are quicker and better organized in combat," Vargas said, cleaning his rifle and mulling over the results of two brief fights the troops had reported the week before. Throughout this region, the contras are operating in groups of 10 or 20 fighters, smaller and more agile than in the past.

The Sandinistas lamented that the contras have gone for the classic guerrilla tactic of avoiding head-on clashes with government troops, waiting to spring surprise attacks.

"They've become professional at making orderly retreats," grumbled del Valle, the doctor, who at times fights as a regular infantryman. Soldiers said the contras are also more thorough now in removing their casualties from battle scenes, rarely leaving behind a body for the Sandinistas to display or a prisoner to interrogate.

During intense warfare in 1983 and 1984, Vargas said, the conservative peasant families in the region gave more help to the contras than to the leftist Sandinistas. In the past two years, the government gave land to peasants willing to move onto cooperatives it controls and forcibly resettled those who were not, leaving many distant hamlets empty.

Vargas said, "We took away the contras' oxygen."

Now the Sandinista strategy is to seek to engage contra fighters in faraway jungles -- away from any peasant population that might sympathize with the rebels.

Unlike earlier campaigns, Rufo Marin officers said, the contras can no longer keep fixed base camps in this region and must stay on the move.

Daily battle logs published by the Nicaraguan Resistance, the main contra grouping, show that most of their actions in recent months were skirmishes with Sandinista Army units like the Rufo Marin in remote hamlets. Even so, the battalion, though continuously in the field on patrol, saw only a handful of full-fledged firefights this year and took less than two dozen casualties, officers said.

Five soldiers died in mid-May in northern Jinotega province in a helicopter downed by a contra missileman. Only one soldier is currently inactive with a combat wound. The contra logs appear to confirm these figures.

Also, the soldiers said no Rufo Marin infantryman deserted from the front this year. Officers conceded that several men were on authorized leaves due to "personal problems." None was absent without leave, they said.

"Two years ago we used to fight the contras every day, sometimes three times a day, long clashes. Now we go for weeks without firing a shot," said a frustrated company commander, 24-year-old Lt. Domingo Ruiz Treminio, at a roadside clearing.

Treminio's restless troops set up one platoon in a muddy cow corral as decoys, hoping the contras would pounce. They ached for a shoot-out, he said. Top commanders tell the troops they are close to annihilating the contras. The sooner they finish, they are told, the sooner they can go home. They seem to believe it.

But sometimes contras have eluded the Sandinistas' outer patrols. In a June 25 ambush on the road that Ruiz Treminio's men now guard, 12 soldiers were killed and a military transport truck was torched, witnesses said.

The contras, trying to focus their challenge on the Sandinistas' weaker flanks, have struck hard at volunteer peasant militiamen, who often are poorly trained. Of 760 deaths the Defense Ministry said its troops suffered this year, fewer than half came from elite units like the Rufo Marin.

To give what they call "ideological cohesion" to their forces, Nicaragua's leftist leaders meshed their party, the Sandinista National Liberation Front (FSLN), with the command structure down to the level of combat units like the Rufo Marin. The young officers in charge are also the party members in the battalion. They derive authority from having fought in the 1979 revolution that brought the Sandinistas to power.

Vargas, Treminio and other officers described themselves proudly as practicing Marxists and their Army as a school for Marxist thought. Vargas called the Sandinista Front "the highest guide of our Army and our society."

Every company has a representative of the FSLN youth organization, like soldier Guzman, who sports a red bandana around his head to show who he is.

"Our job is to make sure orders are obeyed," said 2nd Lt. Danilo Cano, 22, the Sandinista Youth delegate for the battalion. "We have to show we are our battalion's best men." About half the soldiers in the Rufo Marin are Sandinista Youth members, Cano said.

The Sandinistas insisted there are no special privileges for troops who are Sandinistas and no sanctions for not being one. But officers see it as one of their main tasks to "raise the consciousness" of recruits. A draftee who does not subscribe to Sandinista doctrine has no choice but to remain silent.