THE REAGAN administration says it wants, during the next 10 years, a "radical reduction" in offensive nuclear arms. Indeed, such reductions are increasingly admitted to be crucial to the success of the defensive weapons the administration also seeks. Paul Nitze, the coordinator of administration arms control policy, even said in a recent speech that "widespread deployments" of defenses would "accompany" the "global elimination of nuclear weapons" as a means of assuring against cheating.

Many observers have concluded that talk of radical reductions is just talk, unlikely to lead to any actual diminishing of nuclear arsenals. But the new Soviet leadership might be able to force such reductions if it played its cards properly.

The Soviets clearly see the Reagan administration's Strategic Defense Initiative ("Star Wars") as an enormous obstacle to new agreements. But what if the Soviet Union decided to try to preempt and preclude Star Wars deployment through an agreement on reductions of offensive weapons? For example, the Soviets might propose a program of continuing, progressively deeper cuts in offensive weapons that would continue only as long as the United States refrains from field testing or deploying Star Wars systems prohibited by the 1972 treaty banning anti- ballistic missile systems. A well-designed program of annual reductions of 5 percent in each side's inventory of nuclear warheads would lead to substantial reductions in a relatively brief period of time, and could politically tie up Star Wars. This might be the Soviet ace in the hole -- a strategy of Arms Control Embrace (ACE).

Of course to make such a proposal the Soviets would have to overcome their first instinct, which was outlined in Washington recently by Col. Gen. Nikolai F. Chervov of the Soviet general staff. If the United States proceeds with Star Wars, Chervov said, the Soviet Union will respond by adding new capabilities to its offensive arsenal. It's not surprising that a Soviet general would think the best response to new American defensive measures is more offense that could overcome them. Indeed, this has been the traditional approach of both superpowers, and explains why they adopted the 1972 ABM Treaty.

But the Star Wars defense will be a long time building, and to respond to it with new offensive weapons, the Soviets will have to come up with new kinds of offensive weapons and new tactics anyway -- so reductions of existing offensive systems could still be possible.

The ACE strategy is also consistent with the Soviet use of arms control to moderate U.S.-Soviet relations.

It is, of course, precisely this "bear hug" approach that is often feared by hawks opposing arms control, on the grounds that relaxation of tension will really mean relaxation of American vigilance. On the other hand, steady reductions over a long period of time would be an appealing idea, bound to find many supporters in this country, thus making it harder to reject.

Indeed, there has already been considerable support in the this country for the idea of progressive, steady reductions. In 1979, the Senate Foreign Relations Committee unanimously supported a resolution urging Moscow and Washington to pursue " . . . continuous year-by- year reductions in the ceilings and subceilings under the (SALT II) treaty so as to take advantage of the treaty already negotiated and to begin a sustainable and effective process of reductions in strategic arms . . . "

At the June 1979 Vienna summit -- after six months of Pentagon studies of the proposal -- President Carter suggested just such an agreement: a 5 percent per year shrinkage of SALT II limits and sublimits for five years. As he reported later onhis conversations with Leonid Brezhnev, "We both believed that we might conclude a 50 percent reduction in nuclear arsenals on both sides even below the SALT II levels." This would suggest that the Pentagon signed off on at least several years of this percentage-annual- reduction shrinkage of SALT II limits.

It can be shown mathematically that shrinking SALT II by 50 percent would achieve most of the goals set by President Reagan in his 1982 Eureka College speech that called for, among other things, reducing ballistic missile warheads to about 5,000. And while the Reagan administration called the SALT II Treaty "fatally flawed," the principal flaw now cited was the treaty's failure to include disarmament, which a proposal to shrink SALT II would repair.

Moreover, an agreement of this kind would be easy to negotiate -- it only requires agreement on a single percentage. Proposing it would give the Soviets both the moral high ground in this round of negotiations, and a real prospect of heading off the defensive arms race that certainly concerns them. Public opinion in Europe and America would surely be impressed by such an offer.

But a successful negotiation along these lines would not have to be seen as a victory for the Russians. On the contrary, it would give President Reagan an enormous triumph -- he could describe it as just what he intended all along, real reductions. And he could keep his Star Wars research, provided it stays in the laboratories.

If, after far-reaching reductions, the two sides were willing to accept, wanted to purchase and could figure out how to live with some kind of population defense, the president could get that too.

Finally, for those of us who believe that Star Wars puts us on absolutely the wrong road for national security, we would get preemptive arms control instead. With the idea of building a defense gaining momentum in America, there may be no better solution for the Soviets than to hold Star Wars hostage with a reduction agreement of unlimited duration.