The symptoms are unmistakable that Washington is entering one of its relatively lucid intervals for the MX missile debate. Seriousness has not become general, but serious people will dominate both sides of the controversy.

The leading critic of MX -- Sen. Fritz Hollings (D-S.C.) -- was a leading opponent of SALT II.

The crucial question is not whether the Minuteman force -- land-based ICBMs -- is vulnerable. Hollings and the administration agree that it is, because a Soviet first strike could substantially destroy it. Neither is the crucial question whether a land-based component of the deterrent is valuable. Most persons agree that a strategic triad is desirable because it complicates the task of Kremlin planners, preventing them from concentrating their resources on means of degrading the effectiveness of the sea- based and airborne deterrent. And land-based forces still have (although they soon may not have) inherent advantages of control and accuracy (which make them useful against "hardened" Soviet targets).

The crucial question is not whether we can "afford" MX. The nation can afford the price of peace, period. Anyway, even if you budget for something not yet planned -- a protective overlay of ABMs (antiballistic missiles) -- the total MX cost of perhaps $60 billion comes to $6 billion a year over a decade. If MX makes deterrence significantly more stable for such a span, it is a bargain.

The question is not in any simple sense whether the "Dense Pack" basing will "work." To work means to enable a significant number of the 100 missiles to survive a Soviet strike. Each side in the MX debate will have scientists to testify about "fratricide" of incoming Soviet warheads. The "fratricide" theory is this:

MX silos will be so "hardened" that a disarming attack would require saturation by Soviet warheads; but blast effects (heat, shock, debris, etc.) would be such that Soviet attackers would need to plan an incredibly -- indeed, prohibitively -- complex attack sequence, with certain warheads arriving precisely as the blast effects of other warheads are just sufficiently dissipating.

Some critics of MX will say the Soviets can stage such an attack. However, many anti-defense senators and congressmen are precluded (by logic, if nothing else) from making that argument because they have long argued against new strategic programs on the ground that a Soviet first strike on existing strategic forces is prohibitively difficult.

Critics of MX will say "fratricide" cannot be tested. That is true, but the existing U.S. deterrent is woven, in large measure, from untestable hypotheses about how strategic systems will work and how Soviet decision-makers will calculate in certain situations.

The more telling criticisms of MX are more specific. They are that MX's deterrent value is subject to rapid (in perhaps five years) deterioration as a result of Soviet developments (such as ground-penetrating warheads), and that even without such developments the U.S. capacity to "harden" silos sufficiently with steel and concrete is hypothetical and dubious.

The pro-MX side has an inherent advantage because deterrence is less a piling up of certainties than it is the multiplication of useful uncertainties. We cannot know beyond peradventure that "hardening" or "fratricide" would work; but neither can Soviet planners enter a crisis certain that it would not.

The least crucial question is whether MX would violate what critics call "existing arms control agreements." The critics are referring to SALT I, which has expired, and SALT II, which never came close to ratification. Still, the administration argues that MX does not violate the SALT II ban on new fixed-ICBM launchers because MX travels with its launcher. All that is new is the silo into which the launcher -- a cannister -- slips. This interpretation of SALT II is no more legalistic than various Soviet interpretations.

The crucial question about MX or any other strategic program is: how much enhancement of deterrence would be bought for how many years, and for how many dollars per year? Before birds sing their anthems to spring, the debate may have produced a dusty answer: more money for the missile, none for deployment.

But deployment is everything. So a few billion dollars from now, MX may be nothing. Responsible persons who ask, "Can't we do better than MX in Dense Pack?" must also ask, "Can we afford to do nothing?"

The Trident submarine D5 missile, with an accuracy comparable to that of MX, is due in 1989. The U.S. ballistic missile force (as distinct from cruise missiles) may be destined to go to sea. Then America might have a somewhat less vulnerable but also less versatile deterrent. There would be no respite from spending billions to counter Soviet measures, which would be concentrated against submarines. Come what may, there will be no escape from the dialectic of weaponry.