EVERYONE'S worst-case scenario has finally befallen Mikhail Gorbachev. Only its timing comes as a surprise. Many of us have been warning for years that the fledgling reform process in the Soviet Union was far too fragile to survive without committed and sustained support from the West. But now that the worst has actually happened, the West's (and the entire world's) role is still more vital in determining whether this coup prevails.While the Soviet people themselves must act decisively in defense of their own freedom in order for there to be any chance of overturning this counter-revolution, the world community can make a critical difference. By acting quickly to "freeze out" the junta that has seized control of the Kremlin while strengthening links with the many democratic movements and institutions that have grown up during the past six years of reform, we may yet be able to rescue a positive outcome from this long-dreaded event. The success of the counter-revolution will depend largely on its ability to persuade both the Soviet people and the world that it is legitimate. Of course it is not, as many Western leaders have declared. But we have far more powerful instruments than words to undermine its claim to authority. In this second crisis of the post-cold-war era, we once again are given the opportunity we abandoned during the first crisis in the Gulf to apply tough, comprehensive economic sanctions. The UN's structure and history work against strong, unanimous action in this instance. Any Security Council sanctions would be instantly vetoed by the new Soviet regime. So action must be coordinated along other institutional lines. Fortunately most of these are already in place and have proven highly effective in other endeavors. Through the Group of Seven major industrial democracies and the more inclusive Group of 24, actions can be taken to effectively shut down all trade and economic assistance with the new regime that is not essential for medical and humanitarian purposes. The relative handful of nations refusing to take part in such a boycott could not and would not be inclined to make up for the shortfall in resources inflicted by a comprehensive sanctions program enforced by the US and Canada, the European Community and E astern Europe, Japan and Southeast Asia. Coordinated with this isolation of the illegitimate rulers in the Kremlin must be a multitude of initiatives to contact, communicate with, and, where possible, materially assist the disparate democratic movements and institutions within the Soviet Union. Key to the junta's campaign for legitimacy will be its attempt to reassert total control over all systems of communication within the Soviet Union. By every means possible, including shortwave broadcasts and unjammable forms of communication, the West sh ould seek to disseminate prompt, accurate information about events inside the country, including resistance activities. Now that most all media sympathetic to democratic forces have been suspended, these outside links become essential. MEANWHILE, the thousands of economic, political, and cultural links forged in recent years between sister cities and parliaments East and West, between Soviet republics and Western nations, and through Western and Soviet joint ventures should be activated and intensified to create a network of human contacts that is simply too diverse to be annihilated. By every means possible, Westerners with contacts in the Soviet Union should seek to communicate with and visit their Soviet associates and friends. What we need at this point is a nonviolent "invasion" utterly unlike the armed assault so long dreaded by the Kremlin but potentially far more subversive of totalitarian rule. This stick-and-carrot approach should provide one positive incentive for the new regime to reform itself. In announcing trade sanctions, the leading industrial democracies should make clear that the curbs will be lifted only when constitutional processes have been restored, i.e. Gorbachev is returned to the presidency and the Army to its barracks. It may well be that Gorbachev is finished in Soviet politics, but that must be decided by vote, not coup. Perhaps one (admittedly unlikely) outcome of this cri sis could be the first popular election of a Soviet president, monitored by international observers. This second great crisis of the post-cold-war era could be the last if we mismanage it, for in the aftermath we could find ourselves plunged into a new cold war. But by using the cooperative institutions established in recent years, the West may still be able to "veto" this counter-revolution in the Kremlin and assure that the cold war dies a final death.