The helicopter lifts straight to a height of 4,000 feet, presumably beyond the range of the guerrilla riflemen on the mountain slopes below, and within seconds the vulnerability of a small nation at war comes into view.

Two 50-foot-tall steel towers, painted orange to alert pilots landing at the nearby Ilopango Airport, lie tumbled on their sides like tokens overturned on a Monopoly board. The electricity transmission towers have been toppled by dynamite charges placed by leftist guerrillas who have now added El Salvador's economic infrastructure to the list of targets in their war against the military-civilian junta that rules here.

Seen from this helicopter carrying the national Energy Commission's senior engineers and managers on an inspection trip over their power lines, El Salvador's economic war resembles an elaborate cat-and-mouse game that could be crucial to the outcome of a conflict that the Reagan administration has defined as global in importance.

Each of the towers will cost the Energy Commission around $15,000 to replace, a sum that looks like a more impressive drain on the national treasury when multiplied by the 275 times the guerrillas have blown up towers in their campaign against the economy's infrastructure . But the commission's executive director, Francisco E. Granadino, reports that quick repair work and an inventive backup routing system have kept such attacks from shutting down electricity generation to El Salvador's cities--so far, at least.

"They have knocked out enough towers that they could shut the system down now if they wanted to, and were willing to pay the price," Granadino said. "If they don't do it--maybe they don't want to get all our customers mad at them. Who knows?"

To a newcomer during this quick helicopter trip, the surprisingly small scale and mounting tempo of El Salvador's war at first glance look as if they revolve around such things as electricity towers and the competition for least-hated status much more than imperialist conspiracies or detailed Kremlin hit lists of nations.

The guerrilla strategy in a war that is expected to enter a new phase this month with the holding of national elections by President Jose Napoleon Duarte's junta appears to be to try to bleed the economy and wear down the morale of civilians and soldiers on the government's side rather than trying for all-out destruction and chaos.

Whether this is because the Marxist insurgents hope to inherit a functioning economy and a relatively healthy private sector or because they do not want to turn public opinion in the cities against them is a matter of debate among military commanders, diplomats and others here in the capital.

Attacks on El Salvador's power system, telecommunication and transportation networks and an earlier wave of kidnapings and urban terror campaigns that drove many businessmen and new investment out of the country have cost El Salvador $500 million by some estimates. The guerrilla campaign and a climate of fear fanned by violent excesses by right-wing forces have cut economic activity 30 percent in two years, businessmen report.

"There is nothing easier than putting a bomb in an isolated distribution box and knocking out 1,000 telephone lines for 24 hours, until we get it repaired," said Nicholas Caranza, a field grade Army officer who is in charge of Antel, the national telecommunications system. "They say they are knocking out a system that we use to talk to Pinochet in Chile, but all they are doing is knocking out local phone service." He put the cost of repairs to the system in 1981 at $4 million to $6 million.

This is a country in which about one-third of a population estimated at 4.5 million has electricity to begin with. Sixty percent of all energy is supplied by burning wood. There are 60,000 telephone lines here in the capital's metropolitan area (population 800,000), compared with 20,000 in the rest of the country, where the guerrillas recently have concentrated their military efforts and their political indoctrination campaigns to persuade peasants that they are being treated unfairly by the upper classes.

The effect of the infrastructure warfare and the more general Central American economic crisis is mutually reinforcing. Throughout the trip signs emerge of El Salvador's growing inability to finance the kind of changes that could undercut the guerrillas' arguments.

As the helicopter turns north out of San Salvador, it skims above an instant ghost town created by the total collapse of the construction industry here. Dozens of half-finished, low-rent apartment buildings slide past below, walls and floors open to the sky for months now since work stopped.

Five minutes beyond this would-be suburb, the pilot is carefully skirting the Guazapa Volcano, a guerrilla stronghold that can be seen from the top floors of San Salvador's skyscrapers. A quick succession of ridges gives way to the Lempa River, where the Energy Commission has two major hydroelectric generating stations in easy range of the guerrilla units that roam the countryside north of the river. On a hillside above one of the dams stands a microwave relay station damaged by a bomb a year ago.

It would cost $300,000 to repair, Granadino estimates. "But they told us they would just blow it up again, so we haven't fixed it."

Now the main power line swings south and east, often paralleling roads that the guerrillas cut at will as they intensify an effort to isolate the eastern third of the country. In the past two weeks, for example, they shut down a $45 million-a-year shrimp freezing and shipping operation by burning six of the company's trucks on the main road linking the east to San Salvador. They did not harm the drivers, a company official said.

The guerrillas' most impressive trophy is the shattered Puente de Oro suspension bridge that is now tumbled into the Lempa River, a destructive act that has forced a rerouting of road traffic along the main national highway to a nearby railroad bridge that has been modified to take the much-reduced traffic. A government official estimates that it will cost $20 million to replace the bridge.

Flying back toward the glittering high-rises and squalid barrios of San Salvador, a city-state that sits like a giant head on an atrophying body when compared to the rest of the country, the pilot waves toward the San Vicente Volcano on the left, pointing at the multitude of coffee trees that have just been harvested on the isolated but wealthy-looking fincas, or plantations, that climb the volcano's slopes.

"Lots of coffee," the pilot says, beginning to climb another 1,000 feet rapidly. "And lots of guerrillas. Like everywhere now."