There was a note of desperation as well as deepest anxiety in President Reagan's voice when he last talked about El Salvador. ''We believe,'' he said, ''that the government of El Salvador is on the front line of the battle that is aimed at the very heart of the Western Hemisphere, and eventually at us.''
He spoke of weapons from Cuba and other ''Iron Curtain countries'' going to Salvadorean rebels. He said that if ''El Salvador should fall as a result of this armed violence on the part of the guerrillas, I think Costa Rica, Honduras, Panama, all of these would follow.''
The President wants Congress to deal with this startling sounding situation by voting another $60 million for the present government of El Salvador and raising the number of US advisers to the El Salvador Army from 45 to 55 men.
Well, Congress may give the President the extra $60 million and permit 10 more United States military instructors, although grudgingly. But, and here is the central fact in the whole business, it is extremely doubtful that another $ 60 million and another 10 American instructors can make a dent in a politico-military situation in El Salvador which has been going steadily from bad to worse over the past year for the regime the President would like to sustain.
Congress does not share the President's view of the stakes in El Salvador. If it did see it in the stark terms Mr. Reagan used in his speech last Friday at the Commonwealth Club in San Francisco, it would authorize the President to send in whatever military power might be necessary to end the rebellion in El Salvador and protect the present conservative regime.
It could be done. The US has the military power to take over the whole of Central America, if it chooses to do so, at will. There would be some tough fighting in Cuba and perhaps Nicaragua. There would be casualties. But in the end the US would have it all under firm military control. It can be done.
However, it cannot be done with $60 million and 10 more instructors and Congress is in no mood to authorize any more than that, if that much.
The story from the fighting lines in El Salvador gives the reason. Reporters, US military observers, defectors, and prisoners from the rebel lines all agree. The rebels have plenty of weapons, but most of them are not Soviet or Cuban weapons. They are American weapons which reach the rebels partly by capture in battle, but partly also by simple purchase from government troops.
The rebels are learning to use these weapons with increasing skill and success. Government forces are on the defensive over much of the country and losing ground almost daily.
US training of government troops began in January 1982. First important use of such US-trained troops came in August. It was unsuccessful. US-trained units led a counteroffensive against the rebels in November. It achieved nothing and was abandoned. The rebels launched a broad front offensive on Jan. 10. The government forces put in their biggest yet counteroffensive. It stalled and was broken off. Government forces are on the defensive and losing weapons to the attackers at what is described by US observers as an ''alarming rate.''
This is the situation which caused President Reagan to send Jeane Kirkpatrick , his ambassador to the United Nations, to El Salvador on Feb. 3 on a reconnaissance for him. She got back to Washington on Feb. 12. Ever since, the President and Mrs. Kirkpatrick have been talking in doomsday terms about how bad things are there.
There have always been two ways of approaching the problem in El Salvador. One way, consistently favored at the State Department, has been a so-called ''two-track'' plan. It called for pressing the Salvadorean government for reform and a higher standard of human rights (meaning less killing of dissidents) and at the same time negotiating with the political opposition toward a compromise peace.
The other way is to back the ruling regime with US guns and training.
The first way would not necessarily keep Marxism out of El Salvador. The result of a compromise would undoubtedly be a leftist regime, quite possibly Marxist, but not necessarily one associated with Moscow. China and Yugoslavia are both avowed communist countries but prefer friendly association with the US.
The second way seems likely to lead to a regime which is even farther left which would be inclined not surprisingly to look on Cuba and the Soviets as friends and to turn to them for postwar help. For Mr. Reagan that is the worst possible sequel to the story. It will be embarrassing for him if during his presidency one and perhaps more than one Central American country goes communist.
It is probably too late now for him to turn back and take the other road which just might have a less unhappy ending.