The 10-year-old ABM Treaty, which sharply limits deployments of anti-ballistic missle defense systems, has been consistently applauded in the past as the most important arms control agreement reached by the United States and the Soviet Union. It has effectively dampened pressures for deploying a variety of destabilizing, ineffectual and expensive ABMs.

The Reagan administration, however, is reported now to favor for the MX missile a new deployment alternative--"Dense Pack"--which could require major amendments in the treaty, and possibly its renunciation.

The 1972 treaty and its later protcol confine U.S. and Soviet ABM deployments to either each nation's capital or to one of its land-based missile (ICBM) complexes. The actual deployment is restricted to 100 interceptor missiles and a few radars. These limitations effectively prevent either country from trying to defend its opoulation or its ICBM force, except for a very limited defense against accidental or small attacks.

"Dense Pack" would apparently cluster about 100 MX missiles in hardened underground silos over a relatively small area of 10 to 15 square miles. ABM sites would be built nearby to provide the first line of defense against Soviet attacks. Any substantial defense would almost certainly require more ABM interceptors and probably more ABM radars than the treaty allows.

The second line of defense for Dense Pack would count on the explosions by the early attacking Soviet warheads to destroy other Soviet warheads before they could reach their targets. This relies on very uncertain physical phenomena, and makes speculative assumptions about the pacing of the Soviet attack and the timing of any U.S. decision to retaliate.

We should recognize Dense Pack for what it is--a desperate effort to find a basing mode for the now homeless MX missile.

At best, the case for the MX is fraught with uncertainties. It assumes that the increasingly vulnerable Minuteman ICBM must be replaced or supplemented with the new MX missile in some land-based deployment. Other alternatives seemingly rejected by a recent national security directive include deployment of the MX in new, long-flying aircraft, or abandonment of the MX for increased reliance on U.S. bombers, submarine-launched missiles and strategic cruise missiles, as well as any surviving Minuteman. In the calculus of deterrance or actual war fighting, many of these other forces would survive a Soviet strike, and their thousands of warheads and bombs could be used, or threatened, against a wide variety of targets.

Even accepting that a case exists for a land-based MX missile, we must think twice if the deployment involves tampering with the ABM Treaty. Even attempts to amend the treaty in a supposedly minor way can be treacherous.

ABM defense of ICBMs against hundreds, even thousands, of attacking and exploding warheads, which could be accompanied by penetration aids (decoys and chaff), is technically very difficult. The proposed defense might well be much less effective than the planners initially promise. To the extent the system appears to be effective, it could have the perverse effect of encouraging the Soviets to add further to their offensive forces to offset the new U.S. defense.

If the United States seeks an ABM defense, moreover, it would be hard to deny one to the Soviets. In turn, Soviet ABM deployments would probably lead to pressures in the Untited States to increase out offensive capabilities. In the absence of stringent offensive limits -and the fate of the present unratified SALT II treaty is hardly suspicious -a better defense by one superpower will most likely generate more offense by the other.

This problem would be accentuated if the Soviets successfully insisted on deploying an ABM system which, by rapid redeployment or expansion, appeared able to defend cities. Defending cities is even more destabilizing than defending ICBM fields because the United States would have increased doubts about its ultimate retaliatory threat -the ability to destroy a large portion of the Soviet population. Indeed, some of the Soviet ICBMs are near Moscow and other population centers, and so Soviet attempts to defend them would raise doubts about the real prupose of their ABM defense.

In sum, the United States should not rush into some new MX deployment based on questionable assumptions when the most successful arms limitation agreement ever is at stake.