Something important is stirring in the Senate. For the first time in memory a serious and far-reaching arms control proposal has originated within the Armed Services Committee. It has already won the warm endorsement of the chairman of the Foreign Relations Committee and many other senators.

Sens. William S. Cohen and Sam Nunn have proposed a "mutual guaranteed build-down of nuclear forces." Under the plan, the United States and the Soviet Union would each pledge to eliminate from its operational forces two nuclear warheads for each newly deployed nuclear warhead.

At first glance, the concept seems suspiciously simple, but closer examination shows that the build-down idea has great promise as a principle that reconciles the objectives of arms control with the imperatives of military planning. It is the kind of constructive initiative for which those of us who support a nuclear freeze have been working.

One can understand the concept's merit best by considering how it might actually be put into effect. While the build-down scheme is applicable to all nuclear weapons, it would be sensible to treat tactical battlefield weapons as a separate category. Of most immediate interest would be strategic nuclear forces and long- range theater nuclear systems, the weapons currently being discussed in the Geneva negotiations between the two superpowers. All the latter weapons are essentially strategic, in the sense that they threaten critical targets in the heartlands of the two alliances. Altogether, these long-range systems probably carry 12,000 to 13,000 nuclear warheads on each side. Yet, unless negotiated limits are soon imposed, even these large arsenals may continue to grow.

The build-down rule would prevent any such growth. Force modernization would mean simultaneously greater force reduction. Thus, the plan would challenge both governments to move promptly toward the real reductions they profess to desire--to put up or shut up. While the exact pace and composition of such reductions would surely vary between the two sides, the price of new programs anticipated for the next several years could well be cuts of about one-third in operational nuclear warheads on long-range delivery vehicles--a reduction to perhaps 7,000-8,000 warheads on each side. Those numbers are compatible with both President Reagan's START proposal and the Soviet counter-offers.

To verify such reductions, the parties could employ procedures already elaborated in previous strategic arms negotiations. In particular, the so-called "counting rules" negotiated during the 1970s would be invaluable in confirming the numbers of warheads being eliminated. For example, a Soviet SS18 missile would be assumed to have 10 warheads, an American Poseidon missile, 14. Eliminating missiles and aircraft would be necessary to demonstrate removal of warheads from operational service.

In verifying these activities, the Standard Consultative Commission established in 1972 would be indispensable. As the build-down fulfills President Reagan's demands for real reductions, it would also build upon the vital precedents and procedures of prior strategic agreements.

Especially important are the build-down's implications for the American and Soviet military establishments. They would be both liberated and constrained. They would have broad flexibility to choose which new systems to deploy, but they would have to decide whether modernization was worth the price. Presumably, as forces began to shrink, the military incentives would favor emphasizing the most survivable weapons, thereby promoting strategic stability.

One could strengthen those incentives for stability by incorporating some of the ideas that Rep. Albert Gore and others have advanced to phase out the most dangerous weapons, multiple-warhead ICBMs. For example, if a side chose to deploy smaller, single-warhead missiles, it might be allowed to do so at a replacement ratio of two new warheads for three currently deployed warheads. This would still ensure reduction but would favor the less threatening missiles now attracting interest in the President's Commission on Strategic Forces.

Not only is the build-down concept technically and strategically sound; it also is politically appealing. It could go far toward healing the serious breach we have suffered on arms control policy by responding to the fundamental concerns of both those who favor and those who oppose a nuclear freeze. For understandable reasons, freeze proponents have feared that permitting any change in nuclear forces would in fact mean an increase in those forces. Freeze opponents have feared that preventing any change in the forces would undermine stability by perpetuating a structure of increasingly vulnerable and unreliable weapons. Both perspectives are legitimate. The build-down plan offers a unique opportunity to harmonize them. In the interest of stability, some modernization would be permitted, but there would be no modernization without reductions.

More than a decade ago, an overwhelming majority of senators urged the president to head off the deployment of MIRVed missiles by negotiating a ban on flight tests. But we spoke tardily and timidly, and the world grew far more dangerous. Today, with new technologies in prospect and arms negotiations in peril, a vigorous initiative by Congress could not be more timely. The guaranteed build-down could be the organizing principle on which a strong congressional majority could frame advice to the president in terms on which he might be willing to act. It could be the rallying point for a bipartisan coalition linking Congress and the executive in a renewed effort to blend diplomacy and defense. And forging suchha coalition remains the key to effective negotiations with the Soviet Union.