The bottom is beginning to fall out of the administration's heavy investment in El Salvador. The evidence is all around: in the tide of battle (or terrorism, if you will); in Congress; in U.S. public opinion; and most recently in the blessing bestowed upon the revolutionary forces by Mexico and France.

But nowhere is it more telling than in the administration's own response to assorted adverse turns. When the leftist guerrillas take to economic warfare (knocking out bridges and power plants), the State Department complains of a "cynical disregard for Salvadoran non- combatants and a willingness to attempt to destroy the country if they can't overthrow the government." The traditional weapons of insurgency, that is to say, are suddenly unfair.

When the rebels demonstrate growing inside strength, in all corners of the countryside, it's a measure of no thing more than increasing Soviet-Cuban support from outside. So Secretary of State Alexander Haig threatens new, unspecified measures to get at "the source," Cuba, and promises to remonstrate with the mastermind of it all, the Soviets, when he meets with Soviet Foreign Minister Andrei Gromyko later this month.

When American-supplied helicopters break down or get shot up, so that all 10 already on hand often are grounded at the same time, the answer is to send four more.

The policy, we are told, is under "review." But the emerging fixes add up to more of the same: more aid, more concentration on the external content of the conflict, more weight on the outcome as an acid test of American will and ability to contain international communism's encroachments.

Hence the administration's bitter private resentment--and public playing down--of the joint decision by France and Mexico to recognize the various diffuse and sometimes divided revolutionary movements and guerrilla groups as a "representative political force." The motive in both cases has to do with domestic political needs, the line goes: both France's new Socialist President Francois Mitterrand and Mexico's President Lopez Portillo must play to local and international "socialist" sentiment.

Perhaps so--up to a point. But the effect is no less damaging to the administration's cause, or to the position of the Salvadoran government, on that account. For the French/Mexican declaration goes to the central weakness in the current Reagan administration strategy.

It is a strategy that is, at once, very simple and very cynical. It rules out negotiations with the rebels for a share of power they "have not been able to win on the battlefield," in the words of Thomas O. Enders, assistant secretary of state for inter-American affairs. That sounds fair enough; that kind of negotiations, interestingly, is not even the goal of the French and the Mexicans.

And it does not rest on hopes of total military victory. Rather it hangs heavily on elections scheduled for March for a new constituent assembly, and a "new" government. The catch is in who will be permitted to participate. Ironically, the official Communist Party, no real threat, would be "eligible"; the test for the rest of the revolutionaries would be their willingness to "renounce violence," which is to say, to abandon their only influential instrument.

This means that precisely the opposition movements recognized by France and Mexico would be disenfranchised, either technically for lack of formal organization or by their own choice--a choice richly informed by a dismal record of past elections. Under the malign control of the military, only an outcome tolerable to the entrenched oligarchy has traditionally been allowed to stand.

"There's where I have my real problem with the administration approach," says Rep. Stephen J. Solarz (D-N.Y.), one of the leaders of a growing swarm of congressional critics the administration will face this fall. "They're defining eligibility for participation in the election in a way that excludes expressly those forces whose participation is crucial."

That's the point the French and Mexicans are making. Their declaration to the United Nations Security Council seeks not a "settlement," but "a process of political solution," a "new internal order" in which the armed forces "will be restructured and the conditions created for the respect of popular will."

If that's what the administration really wanted, it would embrace the French/Mexican initiative, thereby sharing the burden--and the risk-- and neutralizing the acid test it has unwisely made of El Salvador. But what the administration wants was plainly revealed by U.S. Ambassador Deane Hinton in a recent interview with The Washington Post: "The elections will indicate very clearly that the vast majority of people are in favor of something different than those five or 10 or 15,000 misguided individuals that are trying to destroy the country."

The elections, of course, will produce no such conclusive test. The way the stage is set, they will merely provide synthetic and transitory reinforcement of an increasingly dismal status quo.