AMERICANS worried about the possible leakage of nuclear weapons or weapons-grade nuclear materials from the former Soviet Union increasingly resemble the blind men and the elephant. One group, the optimists, argues that up until now there haven't been any such leakages and points to the public commitments of Russian leaders. Another group, the pessimists, argues that neither of these things provides much comfort, given Moscow's loss of control over many aspects of Russian life. For obvious reasons, all of us hope that the optimists are right. But the dangers posed by even a small nuclear weapon in the wrong hands suggest that we should be paying more attention to the arguments of the pessimists. Only then will we be able to assess what appears to both sides a serious threat. Optimistic views The optimists consist of three distinct groups of people: the Clinton administration, which has linked its foreign-policy fate to the success and reliability of the Yeltsin government in Russia; the arms-control community, which believes that only elites matter in the arms-control business; and isolationists in the Congress and the country, who do not want to face any problems abroad now that we have ''won'' the cold war. Despite their differences, the three groups share a common top-down perspective: Arms control can be left to the small elites. The optimistic argument rests on five assumptions: * There hasn't been any leakage of a weapon or of sufficient weapons-grade materials to make one because we do not have any evidence of a leak. * The proliferation problem generated by the collapse of the Soviet Union was the presence of strategic missiles in Belarus, Kazakhstan, and Ukraine, and this problem has been more or less solved by Russian-American efforts. * The Russian government - whatever its weaknesses - maintains tight control over nuclear weapons and nuclear materials. * Arms control can now be conducted as a government-to-government enterprise. * The commitments of Russian officials to the control of such weapons and materials can be accepted at face value because they share our concerns about the consequences of such leakages. The pessimists - and I find myself in their camp - consist of an entirely different group of people, one seldom heard from in the past during arms-control discussions. Most pessimists have a bottom-up perspective; they are specialists on Russian society who view developments at the societal level as increasingly important for the future of that country, particularly relative to the role of Moscow elites sitting at negotiating tables. The pessimists believe that the optimists are wrong even if they are right - namely, that the new nuclear threat from Russia is greater than anyone wants to admit even if it has not manifested itself up to now. The true threat We pessimists find all five arguments of the optimists unpersuasive: * The absence of information about illegal activity does not mean that it is not taking place. Given the collapse of the Russian and regional police, the weakness of the police forces in many neighboring countries, and the obvious interest of a number of countries and movements in obtaining such weapons, the possible sale of such materials and their transfer out of Russia is all too real. Indeed, the small amount of nuclear materials that have been seized in Germany and widely reported in both Moscow and the West is hardly evidence that materials are not leaving Russia. No self-respecting smuggler would choose to go through Germany when so many easier routes are available. * The nuclear proliferation problem is in Russia, not in the former Soviet republics. Not only is the international community observing everything about nuclear materials in Belarus, Kazakhstan, and Ukraine, but to paraphrase Willie Sutton, Russia is the obvious target for obtaining nuclear materials because that is where most of the materials are. * The Russian government is hardly in control of the country or even of its armed forces. As the Russian media routinely point out, the Russian Army today is poorly paid - sometimes not paid at all - and often not fed, either. Is anyone prepared to argue that a Russian officer who has not been paid for three months - a condition in many units - would not willingly sell something under his control to feed his men and even his family? There is widely reported evidence that Russian officers have done just that. * The end of the Soviet Union has dramatically changed the nature of the arms-control agenda. Prior to 1991, the entire arms-control project focused on control of delivery vehicles rather than on keeping track of warheads or nuclear materials that could make a bomb. That is because noninvasive means could be used to check on the former and because neither side was willing to confront the impact of the latter. Now, the possibility that a warhead - something increasingly small and portable - or nuclear fuel could leak to countries or groups prepared to use them against the West is more real than ever before. And thus the traditional arms-control approach fails to deliver what it promises: a safer world. * The assurances of Russian officials in Moscow - that they share our concerns and perspectives and want to do everything they can to prevent proliferation - can hardly be accepted at face value. Not only do the largely English-speaking elites of Moscow not speak for their own people, whom they no longer control as they did in the past, but these elites, who are in regular contact with Western diplomats, journalists, and scholars, may not even reflect the views of the rest of the Russian elite. Recent Russian polls show that the elite and the masses diverge widely on their support for President Boris Yeltsin and his reforms. Articles in the Russian military press suggest that the generals and admirals who have more control over nuclear materials may have a very different perspective on them than do the diplomats. Thoughtful people on both sides of this debate naturally concede that the other side has a case. On the one hand, we have no choice but to continue to work with Russian elites to try to ward off a disaster; on the other, we have to acknowledge that the Russian government may not be in a position to block the spread of these weapons in every case. What should be done? First, we have to acknowledge that there is a real problem and that there is no quick fix. The search for one may make the situation even worse. Second, we have to continue our contacts with Russian officials in the political, military, and security areas: We need to continue to talk and we need to expand the efforts of America's national laboratories to improve security at Russian plants. Third, we need to increase the presence of the FBI and other law-enforcement agencies in Russia and other countries where nuclear materials could flow. None of these steps will give us any guarantee of complete safety, but we should never forget, especially in this 50th-anniversary year of the use of the first nuclear bombs in Hiroshima and Nagasaki, that a single nuclear explosion, even a small one, could radically transform our world. And this time, not in a better direction.