PARIS - Missile defense? The debate has little to do with national security. An effort costing $50 billion or more will depend on the outcome of a struggle between Republicans' nostalgia and a Clinton/Gore penchant to split any issue.

The Clinton administration is developing a small system designed to intercept the few missiles that a terrorist state might launch against America. In the unlikely case that the administration can convince itself that it will work, the president may decide in the next months to prepare for deployment. More likely, he will pass the decision on to the next administration.

The Republicans want a bigger system. It is not clear why, since they espouse no objective beyond protection against the same terrorist states. Russia and China? Heavens no.

Yet only Russia and China have something to fear from an American anti-ballistic missile.

Russia should fear because the one piece of the Cold War that remains in place is mutual U.S.-Russian deterrence based on the chance that even a surprise attack by one huge arsenal on the other would leave enough surviving missiles for terrible punitive retaliation. But, with everything else in Russia having deteriorated, the number of survivable missiles probably has, too, and the small Clinton defense might suffice to cope with these, thus ending historical Russian deterrence. Even the possibility must cause Russian nightmares.

China has many fewer missiles. The Clinton defense may be able to handle them; the Republican defense almost certainly so, which is probably an unadmitted objective. China is going to increase its missile force anyhow. Faced with an ABM, it will probably do so faster, to nobody's benefit.

America's European allies oppose ABM out of nostalgia for the good old debates of the Cold War. In the course of 30 years the two sides of NATO worked up a panoply of theologies about the circumstances in which the United States would really risk its own existence in using nuclear weapons to defend Europe. The debate continues, even though Europe is no longer threatened by a major atack from anyone.

Europe, like America, can be targeted by terrorists, but it is hard to see what anyone's ABMs have to do with that.

It is hard to see what the proposed ABM systems have to do with terrorist threats. For one thing, of the nations in that category, including Iran, Iraq and Libya, only North Korea is geographically situated to develop a far-reaching missile in a foreseeable period.

North Korea is, in fact, the defining problem. It has actually fired a missile of significant range. But even if North Korea cannot soon be bargained into a verifiable retreat on missile capability, so what? The dirty little truth is that North Korea, or any other terrorist state, not to mention nonstate groups like Osama bin Laden's, will be able to deliver terrorist weapons - biological, chemical, maybe even nuclear - by a number of nonmissile means. All ABM development can do is to spend tens of billions of dollars to force them to develop those alternatives at costs measured in millions.

Why then the passion for ABM? The answer goes back to Ronald Reagan's ''star wars'' vision of an impenetrable shield to protect Americans against all nuclear evil. By the time it had become clear that this was technically infeasible, ballistic missile defense, like Mr. Reagan, had become a Republican icon, particularly for the conservative ''movement.''

One cannot question ABM and be considered a conservative, compassionate or not. It has become a symbol of strong national defense, even though it contributes nothing to that defense. The same tens of billions spent on training, military pay, diversification of the armed forces, even new hardware, would contribute much more.

Why do the Democrats go along? Because in the last eight years the president has done well by going along with anything. Halfway, be it to salvation or to stupidity, defines the middle of the road, and the middle of the road wins elections.

The odd thing, though, is that no evidence exists that ABM is particularly popular or of much interest to American voters. Were Al Gore to break out with a clear statement that missile defense doesn't work, may never work and would serve no useful purpose if it did work, he might be able to shake the weak-kneed image that is losing him the election,

Predictably, he won't.

The writer, an economist, defense analyst and former official in the U.S. executive and legislative branches, contributed this comment to the International Herald Tribune.