Vice President George Bush and Massachusetts Gov. Michael S. Dukakis seem to have squared off on a question that has divided U.S. nuclear strategists for most of the nuclear age: Does deterrence hinge on the details of the U.S.-Soviet nuclear balance, or is it a simple result of the sheer destructiveness of both sides' nuclear arsenals?

Bush's support for a mobile intercontinental ballistic missile (ICBM) and an anti-missile defense based on President Reagan's strategic defense initiative (SDI) reflects the former position. Its proponents, who include most Republican and some centrist Democratic defense specialists, believe that Soviet advantages in some facets of the nuclear balance might afford Moscow diplomatic leverage over the United States or its allies.Dukakis, on the other hand, appears to line up with the more liberal nuclear arms experts, who believe that even the second most powerful nuclear arsenal in the world would wreak such intolerable havoc on its target that notions of a relative nuclear advantage are illusory. Dukakis had flatly opposed both the land-based mobile missile and SDI until early September. He says he might support them, but only under conditions that are so unlikely that he still opposes them in effect.

Behind the debate over a mobile ICBM lies the fact that the Soviets now have enough accurate and powerful missiles to destroy current U.S. ICBMs in their silos while retaining a large enough reserve force to deter U.S. retaliation for the initial attack.

Conservatives say this potential of a Soviet first strike might make the United States and its allies malleable under Soviet pressure. Liberals respond that the whole scenario is fantasy, that no Soviet leader would risk retaliation by the long-range bombers and nuclear submarines that would survive the initial attack on the ICBMs.

The Bush-Dukakis debate over SDI seems to mirror the same philosophical split over the mechanics of deterrence.

The GOP candidate has not expressly repudiated Reagan's "dream" of a nationwide shield that would render nuclear missiles "impotent and obsolete." But he probably would press for a more modest and technically feasible system that would be designed to disrupt the timing essential to a Soviet attack on U.S. missile silos. This view is in line with virtually the entire Republican defense establishment.

Dukakis' skepticism is consistent with the traditional liberal view that a "limited" defense is futile at best - it would not stop all Soviet missiles from getting through - and useless to boot, because the first-strike ICBM scenario is a chimera.

The difficulty with judging the candidates' stands on something as important as strategic deterrence, however, is that the ICBM and SDI issues have been debated in a strategic vacuum, with neither candidate explaining the assumptions that underpin his judgments about specific weapons. And it is these assumptions, which we can only surmise, that will have a profound impact not only on the kind of weapons the next president will build, but also on the kind of arms control agreements he will try to negotiate.