THE PRESIDENT is having a hard time choosing among the various models and makes of the MX he has been shown. Like the typical car buyer today, he is finding each model too expensive for his budget. What's more, he seems to be beginning to suspect that none of the models will deliver what they promise.

The standard model MX, with its drag strips and shelters designed to allow the U.S. to play a giant shell game with the Soviet missilemen, has many technical problems (like, can you really hide the missiles?) and a violent opposition to it from proposed neighbors in the West. Military analysts feel the airborne and submarine versions have even greater technical problems. And if President Reagan does select the standard model designed by the Carter administration, he can expect to have a great deal of trouble convincing the country that it is worth the damage to large areas of the United States and the economy.

There is an attractive alternative, though: buy nothing.

This may be a difficult choice for the president since his election speeches made much of the need to close "the window of vulnerability" that his defense advisers had convinced him would exist in a few years unless he promptly chose the MX missile as a supplement to the allegedly vulnerable Minuteman missile force.

For several years, Americans have been told over and over again by a coalition of ex-government officials and industrialists calling itself the Committee on the Present Danger that they were approaching a "window of vulnerability" in about 1985. It would be a "window of opportunity" for the Soviet leaders when, with a combination of their superior ballistic missile force and large land armies in Europe, they could dominate the world, politically if possible, militarily if necessary. The centerpiece of this scenario is the Soviet capacity to destroy the U.S. Minuteman force of land missiles.

President Reagan and his advisers should carefully examine the emerging evidence that the Minuteman is not nearly as vulnerable to attack as alleged. And, more interesting, even if it were, the U.S. retaliatory forces would end up relatively stronger than those of the Soviet Union after a Soviet attack on the Minuteman. In simple terms, the "window of vulnerability" is a gross deception perpetrated on the president and the American people.

Not only is there no "window of vulnerability" in the offing, but if there were, the MX missile system would not be the way to close it.

I have been following the MX system, examining and analyzing its rationale, since its inception. It has become hard for me to understand why it has any supporters at all. I can only conclude that its advocates have bought the "window" idea, although I can't figure out why.

The scenario assumes the complete destruction of the Minuteman force, with the attendant millions of deaths that would involve. It then projects the Soviets as having so great a nuclear superiority that whoever is president at the time could be bluffed out of retaliating. In other words, the president's advisers are worrying about a "backbone gap."

Analysis of the situation reveals several interesting facts:

It is unlikely that the Soviet Union will be able to eliminate a major fraction of the Minuteman force.

Even if they could eliminate all the Minuteman missiles, it would make no difference strategically. Actually, after a Soviet attack of the kind proposed, the U.S. would emerge with a relatively larger strategic warhead stockpile than before, because the U.S.S.R. would expend more warheads than it would destroy.

The land-based MX system, in the designs I have seen, is very vulnerable to a Soviet attack; in fact, under certain circumstances it would be considerably more vulnerable than the Minuteman force it is designed to augment.

Doesn't this all sound incredible? I had a hard time believing what my investigation was bringing out. Only careful, repeated research and checking has convinced me of its validity.

Much of the MX debate is based on estimates of the size of the arsenals available to each side in the years ahead. I am forced to join in this numbers game to show how unreal the idea of a "window of vulnerability" is. Yet in doing so, I am very uncomfortable because the numerical differences involved really amount to who has the greatest potential for overkill.

I believe that 100 nuclear weapons, neatly placed, would effectively destroy either the United States or the Soviet Union. In these discussions, the debate is over who will have the most thousands; the largest bang is, in my view, irrelevant. But let's look at some of these numbers.

The Committee on the Present Danger estimates that by 1985, the United States will have 12,504 warheads in its strategic fleet, compared to the Soviet Union's 11,728, with the Soviet Union's larger warheads having roughly twice the equivalent megatonnage of the United States. A study prepared for the Congress predicts a somewhat greater U.S. numerical advantage, with 13,904 warheads having 4,894 megatons of equivalent yield, while the Soviet Union is predicted to have 8,794 warheads with 8,792 equivalent megatons.

It should be noted that 85 percent of the Soviet force is on land-based missiles compared to only 25 percent of the U. S. force, which has the remainder of its strategic force on Strategic Air Command jets and submarines. In addition, the United States has a large number of warheads in Europe and on aircraft carriers that could be used against the Soviet Union as part of a retaliatory strike. Finally, our British and French allies have several hundred warheads in their independent strategic forces that could be used against the Soviet Union.

This is the force that the Soviet Union faces and must be able to neutralize by military and psychological means if a "window of vulnerability" is to exist. Clearly, the Soviet strategic force is much more vulnerable to attack than that of the United States and its allies.

The land-based portion of America's missile force is approximately 2,100 warheads on 1,050 missiles. Throwing in some control centers, they offer 1,110 targets. If they could be eliminated, the remaining U.S. force (discounting tactical theater weapons) would have between 10,000 and 12,000 weapons for a retaliatory strike, depending on whether you believe the Committee for the Present Danger or the Congressional Budget Office.

For the Soviets to wipe out the U.S. missile force, military analysts agree, they would have to launch two warheads against each target. If they thus expended 2,220 nuclear weapons on such an attack, they would be left in a relatively weaker position than before. That is, they would use up more warheads than they would destroy, after having fewer warheads than the United States to begin with!

These results are based on the assumption that a Soviet attack could eliminate the Minuteman force. If the attack destroyed only half of the targeted missiles, the U.S. would gain an advantage of approximately 1,100 warheads. There are many reasons why no one can predict the results of such a massive attack with confidence. Calculations of this kind depend on many factors not well known to us, and probably not well known to the Soviets either.

We don't know the accuracy and reliability of the Soviet missile-warhead combination, or the effectiveness with which the Soviet military could coordinate and carry out the difficult, unrehearsed and unprecedently large attack with its need for ballet-like timing to be successful. No Russian does, either. These questions, having no answers, are highly dependent on judgment. The Committee on the Present Danger uses very "optimistic" performance estimates for Soviet weapons and predicts that a Soviet attack would destroy 95 percent of the Minuteman force.

Others, such as the Congressional Budget Office and several of my colleagues here at MIT who specialize in military technology, get much lower levels of destruction. They base their calculations on what they know about U.S. weapon systems, noting especially the complexity of attack, the uncertainty introduced by the fact that the attack flight paths over the magnetic disturbances of the North Pole are not the ones over which Soviet missiles have been tested, and that there is potential interference between attacking warheads, as well as many other technical uncertainties.

I have read the very large literature on this subject. I have chosen performance assumptions that are more generally accepted in the military technology community, and that I regard as conservative. Using this data, I estimate that a Soviet attack might destroy between 300 and 500 Minuteman missiles out of a total of 1,054. Many knowledgeable analysts would say this result is too high -- that is, more Minutemen would survive. But as I have made clear, the exact number does not matter for strategic purposes.

Now for the MX itself. I said earlier that it was surprisingly vulnerable. To start with, the proposed MX shelters, according to information published by the Pentagon, would be able to withstand only 600 pounds per square inch of blast pressure. This compares to 2,000 pounds per square inch for the Minuteman. (This could probably be fixed by spending a few billion dollars more.)

The MX depends for its survival on maintaining "position location uncertainty" (PLU). There is widespread skepticism among technical people about the ability to keep secret the actual location of the real missiles from satellites that can pick up a wide variety of clues.

If PLU is preserved, a Soviet attack employing 2,220 warheads might destroy 80 to 85 of the proposed 200 MX missiles. But if the location of the MX missiles were known, an attack with only 400 warheads might leave only 50 of them.

If the MX is not good for its alleged purpose, then, I have asked myself, has it another purpose? Perhaps it has. Except for its elaborate transportation system, the MX is not unlike the "limited" strategic war-fighting capability that then-Secretary of Defense James Schlesinger tried unsuccessfully to sell the Congress in 1975.

The Congress balked at the concept of creating a nuclear war-fighting capability because of the millions of U.S. civilian casualties that would result. Could the MX be a massive shell game to fool the American people, the Congress, and even President Reagan into building the rejected war-fighting capability?

Every president, after taking office, has had to modify his views from those he expressed during his campaign. If President Reagan choses not to "buy" the MX system, his options will remain open and the future might be a great deal more hopeful for all of us.