THE PROPOSED nuclear test ban treaty has been around so long -- for 50 years -- and has been so shrouded in political foliage that many people have forgotten just what it entails. The current debate about it centers on the Clinton administration's differences with the Russians on the one hand and with the Republicans on the other. But in fact the appeal of the treaty is a good deal simpler and more powerful than the debate indicates. This treaty would put an end to underground nuclear tests everywhere; tests above ground already are proscribed either by treaty or by political calculation. Its merits shine through.

Testing is the principal engine of nuclear proliferation. Without tests, a would-be nuclear power cannot be sure enough the thing would work to employ it as a reliable military and political instrument. Leaving open the testing option means leaving open the proliferation option -- the very definition of instability. The United States, which enjoys immense global nuclear advantage, can only be the loser as additional countries go nuclear or extend their nuclear reach. The aspiring nuclear powers, whether they are anti-American rogue states or friendly-to-America parties to regional disputes, sow danger and uncertainty across a global landscape. No nation possibly can gain more than we do from universal acceptance of a test ban that helps close off others' options.

At the moment, the treaty is hung up in the Senate by Republicans desiring to use it as a hostage for a national missile defense of their particular design. This is curious. The obstructionists pride themselves in believing American power to be the core of American security. Why then do they support a test ban holdup that multiplies the mischief and menace of proliferators and directly erodes American power? The idea has spread that Americans must choose between a test ban treaty and a missile defense. The idea is false. These are two aspects of a single American security program, the one being a first resort to restrain others' nuclear ambitions and the other a last resort to limit the damage if all else fails. No reasonable person would want to cast one of these away, least of all over details of missile program design. Those in the Senate who are forcing an either-or choice owe it to the country to explain why we cannot employ them both.

The old bugaboo of verification has arisen in the current debate. There is no harm in conceding that verification of low-yield tests might not be 100 percent. But the reasonable measure of these things always has been whether the evasion would make a difference. The answer has to be that cheating so slight as to be undetectable by one or another American intelligence means would not make much difference at all.

The trump card of those who believe the United States should maintain a testing option is that computer calculations alone cannot provide the degree of certitude about the reliability of weapons in the American stockpile that would prudently allow us to forgo tests. This is a matter of continuing contention among the specialists. But what seems to us much less in contention is the proposition that, given American technological prowess, the risk of weapons rotting in the American stockpile has got to be a good deal less than the risk that other countries will test their way to nuclear status.

The core question of proliferation remains what will induce would-be proliferators to get off the nuclear track. Certainly a "mere" signature on a piece of paper would not stay the hand of a country driven by extreme nuclear fear or ambition. Two things, however, could make a difference. One is if the nuclear powers showed themselves ready to accept some increasing part of the discipline they are calling on non-nuclear others to accept, so that the treaty could not be dismissed as punitive and discriminatory. The other is that when you embrace the test ban and related restraints on chemical and biological weapons, you are joining a global order in which those who play by the agreed rules enjoy ever-widening benefits and privileges and those who do not are left out and behind.

President Clinton signed the test ban treaty, and achieving Senate ratification is one of his prime foreign policy goals. More important, ratification would make the world a safer place for the United States. Much still has to be worked out with the Republicans and the Russians, but that is detail work. The larger gain is now within American reach.