Scrap the MX and get on with a SICM

President Reagan has a great opportunity to have his strategic cake and eat it too. With one simple proposal he can seize the high ground in United States-Soviet arms control negotiations and begin pulling himself out of the quagmire of the MX missile decision.

The President should announce that the US is willing to scrap the MX missile system - and ban all future deployment of multiple-warhead missiles based on land - if the Soviets will join the US in a genuine phased reduction of existing MIRVed ICBMs to some common low ceiling. If the Soviets are willing to go all the way down to zero, we should be prepared to match them in this destabilizing category of weaponry.

For the US this proposal would mean forgoing both the MX missile system in its current design and the possibility of replacing some single-warhead Minuteman II missiles with triple-warhead Minuteman IIIs. For the Soviet Union this scheme would reverse the buildup of land-based MIRVed missiles and redirect any further effort to smaller, mobile, less theatening single-warhead missiles. For the Reagan administration this proposal has a number of virtues, not the least of which is that it offers a way out of the self-inflicted problem with MX.

By now it is apparent that the MX missile is a strategic orphan, a 190,000 -pound monster missile with no place to call home. Jimmy Carter's system of road loops in the Western desert was technically sound but crushingly expensive, and in any event was unacceptable to ecologists, the Mormon Church, and the local populace. Almost no one likes the current plan to house MXs temporarily in existing Minuteman silos. After all, it was the vulnerability of such silos which gave rise to MX in the first place. Few find any appeal in the future basing mode which Defense Secretary Weinberger is said to find most attractive: putting the missiles on far-ranging, slow-moving, fuel-efficient airplanes.

The plain fact is that the big multi-warhead MX missile was a bad idea from the start. Making a 190,000-pound missile mobile is about as simple as constructing a movable football stadium. Putting 10 warheads on the missile simply gives the adversary a greater incentive to try to find it. In the end, if the MX is ever deployed, it will almost certainly be housed in fixed silos (for want of any reasonable alternative), and it will be protected by an ABM system. This alternative may avoid the expense of a complex mobile basing mode, but it will require the purchase of a ballistic missile defense - and the renegotiation or abrogation of the 1972 ABM Treaty.

At this point President Reagan and his national security team probably wish that the MX would go away, that they could start anew on the design of a mobile missile system. What the US really needs is a whole new missile--smaller, cheaper, and more mobile than MX. In mobile missiles, big is bad and small is beautiful.

The idea of a small intercontinental ballistic missile--''SICM'' for short, or perhaps ''Midgetman''--has been around for some time. A SICM could be designed to weigh about 20,000 pounds, about a tenth the weight of the MX, and carry a single warhead in contrast to the 10 on MX. It could be carried on a truck or railroad car, giving the smaller missile a versatility and mobility which would be impossible for MX.

Scrapping MX in favor of SICM makes good sense on its own merits, but getting something from the Soviets for such a move makes the idea even more attractive.

Right now the Soviets have over 800 MIRVed ICBMs, 250 more than the US. These are the most destablilizing weapons in the Soviet inventory. It was these weapons which gave rise to fears of a Soviet preemptive attack. The Soviets now have enough warheads in their land-based missile force to direct two at every American ICBM, and still hold a comfortable force in reserve. But all is not well for the Soviets. They have their own window of vulnerability opening on the horizon.

Over 70 percent of Soviet megatonnage resides in fixed-silo ICBMs. If the US ever does deploy MX it will give the Soviets the same concern about preemptive attack which they are now giving us--with one big difference. While the US worries about losing a quarter of its megatonnage to a Soviet first strike, the Soviet Union would stand to lose nearly three-quarters of its nuclear might if the US went first. The anxieties thus generated might in a crisis tempt the Soviets to launch their missiles first, thinking their choice is to ''use 'em or lose 'em.'' Given this impending problem, it is in the US interest to encourage the Soviets to shift to smaller, mobile missiles with single warheads. We would feel more secure; they would feel less threatened. Strategic stability would be greatly enhanced.

There is a problem with this proposal, but it is not an insurmountable one. The mobility which makes a SICM-type missile so attractive for strategic purposes also makes it a problem for arms control. Since the missiles are small and easily transported, the total number of such missiles would be difficult to monitor. Without high confidence in verification, the US would almost certainly not accept a limit on the total number of strategic nuclear delivery vehicles.

Verification will be a real problem in the future even if SICMs are never deployed. Cruise missiles pose an even greater challenge to detection capabilities. Despite this problem, it is not inconceivable that US and Soviet arms negotiators could work out some cooperative measures for monitoring the production of new hard-to-find weapons like cruise missiles and SICMs. In any event, a major reduction in the most destabilizing sub-class of strategic weapons, land-based MIRVed missiles, is important in its own right and should not be delayed until Moscow and Washington can conclude another comprehensive agreement covering all strategic offensive forces.

Much has been written about the alleged ''dangers'' of arms control, the tendency of earlier administrations to put arms control ahead of strategic doctrine. This proposal puts arms control in the service of defense strategy. A ban on new land-based MIRVed missiles and a phased reduction in existing inventories of such missiles would use arms control to further US strategic interests - and would produce a more stable, less threatening strategic balance between the two nuclear superpowers.