It is a tiny wretchedly poor back water that for generations stood outside the mainstream of world attention. Yet El Salvador -- seemingly the prototype Central American "banana republic" -- has become the arena where the Reagan administration is moving to draw the line against insurgency supported by the Soviet Union and its communist allies.

Just as then-president Carter's outspoken championing of human rights four years ago put an indelible stamp of identity on his adminisration's foreign policy, President Reagan's top-priority concern with leftist insurgency in the Third World already has been taken as a metaphor for the instincts and directions that his fledgling administration will follow.

In the process, El Salvador, a Massachusetts-sized country of only 5 million, has become the improbable center of a globe-girdling web of policy moves aimed at holding the Soviet Union to "an international code of conduct" and testing the willingness of America's allies to support its get-tough stance.

Inevitably, this rush toward increased U.S. involvement in El Salvador's bloody civil war will provoke a major debate in this country about whether Reagan and Secretary of State Alexander M. Haig Jr. are orchestrating the intelligence at their disposal to whip up support for their hard-line attitudes and push the United States into what many liberals fear could become a Central American miniversion of Vietnam.

For the past week, the administration has been spelling out to foreign governments and U.S. congressional leaders the evidence it has compiled to prove its charge that Cuba, acting as Moscow's surrogate, has been smuggling massive amounts of arms through Nicaragua to the leftist guerrillas seeking to overthrow El Salvador's ruling civilian-military junta.

Monday, the State Department plans to reveal part of this evidence to the American public as the next step in its evolving policy of putting what one official calls "a clear and unmistakable shot across the bow of Moscow and Havana."

The aim, U.S. officials have said, is to demonstrate that the administration will use all its resources, including increased military and economic assistance, to prevent a leftist military takeover of El Salvador. More importantly, the decision to make a stand there is intended as a signal that Washington is prepared to use similar tactics against communist-supported insurgency movements elsewhere in Latin America and even in more remote areas like Africa and the Middle East.

So far, the reaction has been largely of the approving kind enuciated by Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee. He emeged from a state Department briefing for congressional leaders last week to say,"Haig is right. This is the place to draw the line."

But, as the administration is keenly aware, lurking in the background is a wide variety of potential critics waiting to scrutinize the evidence of Cuban complicity with a very skeptical eye. Many of these critics, in Congress and elsewhere, also can be expected to argue that the administration in its eagerness to contain communism, is running the risk of repeating the same mistakes that were made in Vietnam.

Specifically, they will contend that the United States is allying itself with an authorization government that is widely suspected of having ties to rightist extremists and have no popular support among El Salvador's peasant masses, and that efforts to prop it up are likely to lead to ever-deepening American involvement.

In addition, the large constituency that was attracted to the banner of human rights during the Carter years is certain to argue that the administration, in taking a relatively nonchalant attitude toward the mass murders and excesses of the Salvadoran rightists, will be playing into the hands of Moscow by depleting the reservoir of goodwill toward the United States that the Carter policy built up in the Third World.

Others, including some foreign governments, are likely to cite the risk that the administration, if it finds the guerrillas harder to subdue than expected, might escalate the situation by taking direct measures such as a naval blockade of Cuba, thereby elevatiing what most of the world now sees as a tragic regional conflict to an international crisis. Haig gave an ominous hint of that possibility when he briefed allied ambassadors last week.

According to a leaked transcript of his remarks, he said of Cuba, "I wish to assure you we do not intend to have another Vietnam and engage ourselves in another bloody conflict where the source rests outside the target area. . . . We do not anticipate dealing with that situation in the historic sense of what we did in Vietnam. We are studying a number of alternatives."

The opening salvos of this impending debate will be heard after the administration makes public its intelligence information tomorrow. While a verdict on the quality of the evidence cannot be made until it is subjected to inspection, the advance indications are that the administration will win that round handily.

Well-placed sources familiar with the evidence point out that most of it was collected during the final days of the Carter administration. They add that even before Reagan took office, there was virtually unanimous consensus among senior Carter administration officials, including some like former secretary of state Edmund S. Muskie who previously had been chary of the arms-smuggling charges, that since early December the Cubans have been masterminding a large flow of weapons through Nicaragua.

As one source said: "The evidence is so incontrovertible that if Carter had been reelected, he would have had to respond with some kind of crackdown. When it's laid out publicly, no one's going to be able to argue convincingly that the Cubans aren't involved and that the guerrillas are simply native fredom fighters untainted by outside influence."

On the question of whether a successful counterinsurgency campaign can be mounted in El Salvador, many critics are likely to make comparisons with neighboring Nicaragua, where leftist guerrillas won power two years ago.

There, the guerrillas triumphed because virtually every segment of Nicaraguan society turned against the long, despotic rule of the Somoza family. The Somazas' loss of popular support was so widespread that not even a big infusion U.S. help -- denied by the Carter administration on human rights grounds -- could have kept that regime in power.

However, Nicaragua was not typical of the Cuban-supported insurgencies that have cropped up throughout Latin America over the past two decades. In other countries -- Guatemala, Columbia, Venezuela, Bolivia and peru -- the guerrillas never won the popular support necessary for victory, and the local armed forces, bolstered by U.S. weapons and training, eventually crushed them.

Despite the undoubted unpopularity of the Salvadoran government, there so far is little sign that the insurgents have succeeded in "winning the hearts and minds" of a peasantry left apathetic and intimidated by centuries of oppression. In fact, the so-called "final offensive" mounted by the guerrillas last month was a failure both militarily and in terms of triggering any mass outpouring of support for their cause.

As a result, the administration is hopeful that there is still enough time to crush the guerrilla movement before it does start gaining ground politically. While stressing that this is the first priority, U.S. officials, increasingly sensitive about charges of embracing a dictatorship, insist that Reagan and Haig intend to continue the Carter policy of prodding the junta toward making the reforms that will give it a popular base of support.

If the lid can be kept on long enough to prevent antigovernment sentiment from building up to a Nicaragua-type explosion, U.S. policymakers are confident that El Salvador's tiny size and lack of jungle terrain will permit successful counterinsurgency operations without the massive American military involvement that took place in Vietnam.

It is now virtually certain that U.S. military assistance to El Salvador will be increased, and that means more American military personnel (25 are there now) will be required for training and maintenance. With even a small increase in personnel, one U.S. official admits, "there's an increased risk that someone will get shot and touch off an uproar about it being Vietnam all over again."

"We're not talking about tanks and missiles," the official noted. "There's not even any plan at this stage to send American combat advisers there. What's involved is small arms, ammunition, maybe some helicopters and coastal patrol boats. It's a game we can buy into relatively cheaply, and there's clearly a consensus emerging within the administration that the risks are worth the potential payoff."

In short-range terms, that assessment probably is correct. By putting its weight behind the junta, the administration most likely will be able to block a leftist military victory. By putting pressure on the allies, it probably also will win some expressions of qualified support, particularly from Western Europe where most governments tend to view the situation as remote from their major concerns.

Much less clear, however, is whether these results will have the major, long-haul impact that the administration hopes to extract from its policy. Still very much in question is whether a show of toughness in a tiny Central American country -- one that is securely within what even Moscow concedes is a U.S. sphere of influence -- will impress the Soviets sufficiently to make them mind their manners in other parts of the world.