The United States is prepared to counter any increase in the military strength of leftist guerrillas here with a comparable rise in U.S. military assistance to the Salvadoran government, U.S. Ambassador Deane Hinton said in an interview.

"We don't want to do it," Hinton said during a lengthy session yesterday as he sat in his office behind the 20-foot-high walls and sandbag gun emplacements of the U.S. Embassy. He said he hoped U.S. military aid, which totals $35 million in the current fiscal year, would actually be diminished.

But, he said, should the aid the administration says the guerrillas are receiving from the Soviets or others be stepped up, he "would recommend" a boost in U.S. military aid.

"Increases and decreases in the mix are a function of the formula needed to make sure the Carpios of this world" -- a reference to guerrilla leader Salvador Cayetano Carpio -- "do not win," Hinton said.

An essential element in the formula Hinton outlined for the defeat of the left is the election of a constituent assembly scheduled for March, which he said will resolve the question of which side has popular support in the country.

Although the opposition Democratic Revolutionary Front has said repeatedly that it will not participate in the elections unless they are preceded by negotiations, Hinton and a member of the Salvadoran electoral commission said that the commission has secretly met with some Front members.

Some U.S. policymakers in recent weeks have expressed hope that the opposition, which they believe is divided between more moderate political components and hard-core Marxist guerrillas, will fragment and provoke a lessening of the international political sympathy that the left currently receives from a number of Western countries, as well as the Soviet Bloc.

Hinton said he doubts that such fragmentation will occur. But he said he believes it unlikely that the 4,000 to 6,000 guerrillas in the country ever would lay down their arms voluntarily, no matter what their political allies agreed to, and that hard-liners among them, like Carpio, would rather die than admit defeat.

The ambassador maintained that recent increases in guerrilla activity are a sign of the rebels' desperation.

"I think it is perfectly clear with the offensive against the power system, the transport system, the communications system, that the guerrillas, having discovered they can't win, changed their strategy and they are out to destroy the country," Hinton said.

"They're not going to succeed," the ambassador continued. "Eventually, bit by bit, the Army will increase its effort and slowly this will come to an end. The country will be worse off. Lots of guerrillas will be dead. And for what end? They the guerrillas are not going to win. It's hopeless."

Hinton did not say what if any increases in military aid are currently necessary to counter what the embassy believes are mounting arms shipments to the guerrillas from Cuba and other sources. But the ambassador did note some current supply problems of the Salvadoran Army itself.

Of 10 military helicopters on loan from the United States, the ambassador said, only three were able to fly as of Tuesday and at one point last week not one was airworthy. Damage from guerrilla groundfire, constant use and delays in the delivery of spare parts are the essential problems, he said. "They're lucky they haven't lost any," he concluded.

Salvadoran President Jose Napoleon Duarte, who heads the civilian-military coalition government, already has said that the government needs more helicopters and communications equipment to increase military effectiveness against the guerrillas.

Meanwhile, the town of Perquin in northern Morazan, where the guerrillas have retained control for more than a week, remains the scene of heavy fighting.

Hinton described Perquin, until recently a village of more than 3,000 people, as "a speck on the map that doesn't amount to a hill of beans." The Army, Hinton said, could retake it any time it wants, but is currently distracted by widespread sabotage against the nation's electrical power system.

According to Hinton and Ernesto Rodriguez Rivas of the Salvadoran electoral commission, some members of the National Revolutionary Movement, a social democratic party that has lobbied worldwide for support of the insurrection and whose founder, Guillermo Ungo, is the president of the Revolutionary Front, have approached the electoral commission to learn the regulations covering the March balloting.

Rodriguez Rivas said a meeting took place in May, but the socialists made no commitment about participating in the elections. He would not say exactly what members of the party were involved in the discussions.

"There are all kinds of those people," Hinton said in reference to the moderate leftists, "who would like to come back in out of the cold."

Members of the National Revolutionary Movement now in exile have said they believe the elections will be fraudulent and are designed to give an air of legitimacy to a new government so that it can increase its military activity against the insurgents.

These leftist politicans also say they would be killed if they returned to campaign for office, just as other leftists have been killed in the past by right-wing death squads believed to be allied with the government.

Hinton said he believed "personal security is probably the most difficult problem of the elections."

"Who would kill them the leftist politicians ? The left -- the same extreme left people who would call them traitors," Hinton said, while acknowledging that death squads would also be "a problem."

"There are a lot of people in this government who understand" the need to bring the moderate left into the elections, Hinton said, "and a lot who don't understand it."

"We will try and the government will try to create conditions where it will be conceivable that they will participate," said the ambassador.

Asked about the murders of three American nuns and a lay worker last December, for which six members of the Salvadoran National Guard are being held as suspects, Hinton said that a conviction is not impossible, but not probable.

"These men are in a legal sense innocent until proven guilty. Now I personally am convinced -- but my personal convictions don't have much to do with it -- that the guardsmen are guilty. But if you can't prove it in a court of law, under the rules, and they are released because the jury or the judge finds them innocent or for some other reason, I will regret that but clearly it has to be accepted," Hinton said.