PRESIDENT REAGAN is about to make a costly mistake. He and his advisers have spent an inordinate amount of time--time that would have been better spent talking about arms limitation--discussing weapons, particularly the MX missile. By all the most recent accounts, the administration is leaning toward a scaled-down version of the mobile missile system first proposed by former president Jimmy Carter and derided by Reagan the candidate. Just as the Carter scheme was ill-conceived from the start, the smaller Reagan plan is equally misguided.

Carter originally proposed shuttling 200 giant MX missiles among 4600 shelters burrowed into the deserts of Utah and Nevada. The rationale for this nuclear shell game went something like this: The Russians now have enough warheads to destroy our 1000 land-based Minuteman missiles in a nuclear first strike. With the MX system, the Russians would have to hit 4600 targets to ensure the destruction of all the mobile missiles. Thus, our land-based nuclear deterrant would remain intact, and everyone could rest easier.

Everyone, that is, except the people of Utah and Nevada who didn't like the idea of having a subway for nuclear bombs in their backyard. Among those who opposed this basing scheme were Sen. Paul N. Laxalt (R-Nev.), Sen. Jake Garn (R-Utah), and the Mormon Church. With some of its best friends fast turning into foes, the administration last spring started looking at other options. It has apparently returned to the land-based MX, with the following alterations: The number of missiles and silos has been halved, and the system reportedly includes anti-ballistic missiles (ABMs). This plan, like the original, is indefensible on several counts.

Fundamentally, the MX is a preposterous idea, a "mass transit system for missiles" that would use resources on an unprecedented scale and destroy vast areas of the desert. But more disturbingly, the need for a mobile missile is based on an unsound premise. That the Soviet Union can destroy all our Minuteman silos in a first strike is a highly suspect assumption, based on as-yet unproven theories of missile trajectory and accuracy. (Neither the United States nor the Soviet Union has ever test-fired a missile on a North-South route, similar to that which both countries' bombs would have to follow.) If enough Minutemen can survive a first strike--as is most probably the case--the argument for the MX collapses.

Even if one accepts the Reagan point of view that we now live with a "window of vulnerability," enabling the Russians to knock out our land-based bombs, the mobile MX still adds up to a colossal mistake. Assuming the Soviet Union can destroy our 1000 Minutemen, it could eventually gain the capacity to knock out 2300 MX sites. Some estimates show that the USSR would have that ability before completion of a mobile MX system, placing the United States in the same "vulnerable" position it is in now.

That is where the ABM comes in. Proponents say it would significantly reduce the Soviet Union's chances of destroying all the MX missiles on a first strike. But to deploy ABMs would scuttle one of the most significant achievements of arms limitation talks--a ban on ABMs--and would prompt a costly ABM race. Relying on ABMs would also defy good sense. They were abandoned in the first place because they lead to destabilization of the delicate strategic balance and because they are unreliable; ABM technology has reportedly not advanced to a point where an ABM system would be effective.

So where does all this leave the MX? Perhaps, realizing the weight of logic, the Reagan administration wants the mobile missile only to use as a negotiating device in some future arms limitation talks. But such talks are a long way off, and the $100 billion MX would be an expensive bargaining chip. If the Reagan administration is committed to the MX missile, as it seems to be, it will have to come up with a better argument for deploying it than the oft-touted "window of vulnerability." For now--especially since the administration is looking for new ways to reduce spending--the best plan for the MX is to forget about it.