Without United States military support, the government of El Salvador would fall to guerrilla forces within a few months. And a guerrilla government here would be far more repressive than the Sandinista government in nearby Nicaragua.
That is the two-point conclusion, predictably, of Salvadorean and US observers on the right and center of the political spectrum.
More surprisingly, however, that also appears to be the consensus of the left-wing intellectual community here.
Even as a congressional subcommittee voted April 12 against President Reagan's request for $50 million in supplemental military aid to El Salvador this year - and slashed his military aid request for fiscal 1984 and 1985 from $ 86.3 million to $50 million - observers here say a complete withdrawal of US military support could produce bloody results.
First, it could cause panic among the military, stiffening its repression of suspected left-wingers. It could next produce major and costly battles between guerrillas and the Army. And finally, it could lead to further bloodshed as a new guerrilla government purged the nation of suspected right-wingers, in the view of military observers here.
How to avoid such a situation? Left-leaners call for a winding down of US military aid, accompanied by negotiations with the guerrilla forces. Moderates and most rightists call for increased military aid, along with elections among a broad range of parties.
But on one thing all agree: A left-wing government in El Salvador would provide tremendous encouragement to the Sandinistas in Nicaragua, who are actively supporting the Salvadorean guerrillas. And that might further roil the social waters in other Central American countries and even in Mexico.
These days, the war here is in something of a stalemate. Western estimates put the guerrilla forces at between 5,000 and 7,000 men. The Salvadorean Army now stands at about 21,000, with National Guard and Treasury Police forces contributing another 10,000.
Against such odds, observers here say, the guerrillas cannot prevail - although they can continue to destroy electrical installations, ambush Army patrols, and hold outlying towns for a few hours or even days at a time.
But neither, says a military observer, can the Army prevail - since, he says, experience shows that success against guerrillas requires about a 6-to-1 advantage in conventional forces.
''If we had another $60 million, plus changes in the military leadership, we could turn this around,'' he says.
The money, he says, would be largely used to train and equip one more ''immediate reaction'' battalion and four to six small (350-man) cazadores (''hunter'') battalions.
In addition to new troops, however, Western observers here see the need for significant changes in the Army in several areas:
* Leadership. Defense Minister Jose Guillermo Garcia has been openly criticized by some US officials for running the war more for political than military advantage. In the nation's 14 provinces, a Western observer says, ''You've got 14 warlords'' - commanders who all, by virtue of friendship and political pull, report directly to General Garcia rather than through a command structure responsible for coordinating the fighting.
Now, however, there are widespread rumors that General Garcia may retire within the next few months - to be followed, perhaps, by Col. Carlos Eugenio Vides Casanova, whom a Western observer describes as ''everybody's second choice.''
Top officers of the Salvadorean Air Force met Thursday to discuss possibly defying Garcia. Commander Juan Rafael Bustillo reportedly will not obey Garcia's orders if the defense chief does not step down by today. Garcia, however, insists he will stay on until presidential elections are held.
* Strategy. The Salvadorean Army, which has resisted using small patrols and taking offensive actions against guerrillas, operates largely in a ''stand and fight'' manner - a pattern permitting the concentration of major decisionmaking in the hands of the few commanders. But ''this is not a generals' and colonels' war but a squad-leaders' and platoon-leaders' war,'' a military observer says. He says the military needs smaller, quicker units willing to chase the enemy rather than simply guard fixed installations.
''A national campaign plan and a reorientation of the military is essential, '' he adds, noting that the Army has had its greatest successes in areas where commanders are willing to follow such new strategy.
* Manpower. Even the platoon-leaders, however, tend to be young and inexperienced. With unemployment high, the Army has no difficulty recruiting soldiers. But it is encountering serious shortages of officer candidates. Manpower is a problem at the upper levels as well: In the past, some observers say, commanders have been chosen for their political loyalties to those above them - and not for their military acumen, which might pose a threat to those already in power.
* Human-rights concerns. The population of San Salvador appears to be more afraid of government forces than of the guerrillas - a legacy of the months of repression following the coup of Oct. 15, 1979. US efforts to help professionalize the Army are making some headway, but the idea that an officer is above the law is still strongly entrenched.
But US advisers are prohibited from working with the nation's substantial police forces, which (along with right-wing vigilante ''death squads'') are seen as the focal point of human rights violations. Members of the National Guard, for example, have been implicated in the still-unresolved murders of four American churchwomen in December 1980.
Western observers, however, are quick to point out that there has been progress. They point to the success of the Navy in intercepting gunrunners from Nicaragua across the Gulf of Fonseca, to the current group of 180 officers now being trained at a US facility in Panama, and to the work of the Ramon Belloso Battalion trained in the US at Fort Bragg, even though the battalion sustained significant casualties during a guerrilla ambush in Morazan Province March 30.
There is concern, however, that the guerrillas are retaining their strength. The official line here is that the rebels have little support in the countryside and are unable to recruit. But for each of the past several years their strength has been put at around 5,000 men - and each year the Defense Ministry estimates that 4,000 have been killed.
''The fact is,'' says a left-leaning academician here, ''that you cannot feed 5,000 men three times a day simply by harassing the local population.'' He says simply sustaining such a force requires ''some sort of popular support.''