MIKHAIL GORBACHEV, speaking as negotiations resumed in Geneva, put a large package of arms control proposals into public orbit. One proposal looks ambitiously to timetables for phasing out all the nuclear weapons in the world by the year 2000 and for halving Soviet and American arms that can reach each other's territory along the way. A second looks more modestly to eliminating medium-range missiles in Europe. A third looks guardedly to a full, mutual nuclear-test ban, to be policed by unspecified "international procedures" and "on-site inspections where necessary" in addition to the usual "national technical means." But there is, for Moscow, a crucial condition to all these possibilities: President Reagan must forgo the "development, testing and deployment" of his Strategic Defense Initiative -- Star Wars.

Mr. Gorbachev and his comrades, in other words, and Mr. Reagan, are true believers. The Russians fear and the president hopes that SDI can ultimately raise a leak-proof nuclear umbrella. Rejecting the Reagan disclaimers of peaceful intent, the Russians believe that if SDI works the United States will have a shield behind which to threaten them, and if it doesn't the economic, political and strategic costs to them will still be immense.

The Soviet leadership seems to doubt President Reagan will give up the project. But it's doing its best to generate public pressure in this country and Europe and Japan to make him do so. It's appealing to the great popular anxiety about nuclear war from which Mr. Reagan's vision of SDI also flows. The potency of its political program should not be underestimated.

SDI apart, the gap between the Soviet and American positions appears to be narrowing. Moscow's priority is to reduce the threat of superior American technological capacity; Washington's is to reduce the threat of superior Soviet first-strike capacity. As always, it's a tough match. The two sides are working toward the tentative deadline of a second Reagan-Gorbachev summit later this year.

SDI remains central. Sometimes -- the Russians are not consistent here -- it seems that they would stand still for SDI research. As a research program conducted in the laboratory under the terms of the Anti-Ballistic Missile Treaty, the American search for a space-based strategic defense would merely be following in the tracks of the long-term Soviet search for a land-based strategic defense. Their tense relations of recent years, however, have so far kept the two powers from agreeing on exactly what those terms ought to be -- on whether, for example, the treaty permits the United States' recent underground test of a nuclear component of one variant of the ostensibly non-nuclear SDI.

The Gorbachev statement, then, is bound to increase the pressure on the United States to address more precisely the Soviet complaints about SDI. The statement makes a certain play for the crowd -- especially in its utopian promise of an nuclear-free world and in its appeal for a test ban, which is something of a distraction from the priorities of negotiating arms reductions and strengthening the stability of the forces that remain. In other aspects, however, Mr. Gorbachev offers a program that, if negotiated, could yet bring major strategic gains for both great powers. The Geneva negotiations should tell whether these gains are actually within reach.