IF THE SOVIET UNION really wants to stop Star Wars, it should make Ronald Reagan an arms-control offer he can't refuse -- an offer that would give the United States the same benefits as Star Wars at a much lower price.

The centerpiece of this agreement would be a Soviet offer to abolish the most threatening weapon that the Strategic Defense Initiative would be designed to knock out -- the weapon the Reagan administration says it fears most: the SS-18, a behemoth "heavy missile" that carries 10 highly accurate warheads capable of destroying our Minuteman missiles in their silos.

In exchange, we would agree to limit SDI.

Reducing the Soviet threat to our missile force by abolishing the SS-18 "would be the functional equivalent of SDI," says Lt. Gen. Brent Scowcroft, a former national security adviser in the Ford administration.

"From a negotiating standpoint, this tradeoff would be a sensible way to move," says former SALT II negotiator Paul Warnke. "It would allow the Reagan administration to accomplish what they say they want to accomplish -- a survivable ICBM force -- much more cheaply and reliably than SDI."

This sort of tradeoff -- in which each side stands to gain at least as much as it gives up -- is the basis of any successful negotiation.

Such a "grand compromise," as Strobe Talbott of Time magazine has called it, could be the crowning achievement of the Reagan administration. It would enhance American security in the ways that hardliners have urged for the past decade -- but without the cost, military dangers and political headaches of Star Wars.

In making this bargain, the Reagan administration would be recognizing that the most that can be expected from SDI in this century is a system of partial defense, one that might be able to defend our missile silos, but couldn't fully defend population centers.

To see how a new arms-control deal might work, let's take an imaginary walk in the woods with the Soviet negotiator in Geneva, Viktor Karpov, and his American counterpart, Max Kampelman.

Kampelman wants to reduce the Soviet threat to America's nuclear deterrent, particularly our silo-based missiles. For this reason, he strongly supports SDI, seeing it as an important way to reduce our vulnerability. Kampelman wrote last year that one of the reasons to deploy space-based defenses is that they "could effectively contend with the menace of the Soviet SS-18s."

Kampelman also favors sharp cuts in the throw-weight of Soviet heavy missiles, such as the SS-18s, and has made a proposal that would accomplish this goal. (See chart.)

As for Karpov, his main goal in the Geneva arms-control talks is to limit SDI. This month, in fact, he proposed sharp cuts in Soviet land-based missiles in exchange for a pledge that both sides would continue abiding by the 1972 Anti-Ballistic Missile treaty for at least 15 more years. He also proposed amendments to the ABM treaty (such as a redefinition of the word "development" and a new definition for the word "prototype") that would have the effect of banning all but the most basic laboratory research on SDI.

As Viktor and Max stroll through the woods outside Geneva, they realize that with some clever bargaining, they both can accomplish their goals.

Here's how the compromise works: Both sides agree to reduce their arsenal of nuclear warheads to 6,000, of which no more than 3,600 could be based on ICBMS (numbers that were included in an initial Soviet offer last year). This is about half what the Soviet ICBM warhead total would be in 1990 under SALT II. What makes this offer hard to refuse is Karpov's promise that in meeting these new levels, the Soviets will scrap their SS-18 force entirely (and allow the United States to verify that the missiles have been destroyed). This means they lose 3,080 highly-accurate, silo-busting warheads.

The Americans, in return, agree that for 10 years, they will limit SDI to a research effort, with no operational testing or development. (We might also agree to limit our potential "first-strike" weapon, the MX.).

This deal is attractive to our hypothetical American negotiators for a simple reason: they realize that it would be vastly more expensive, and perhaps technologically impossible, to devise an SDI system of defenses by 1995 that could accomplish the same goal of neutralizing 3,080 warheads. Abolishing Soviet warheads at the stroke of a pen is much easier than shooting them down.

In real life, giving up SDI will be very difficult for the Reagan administration. Explains Michael Mobbs, assistant director of the Arms Control Agency:

"If you conceive of SDI only as something that you do to enhance the security of the ICBM fields, then obviously you have to consider all the ways you can enhance this ICBM force. But I don't think we should consider SDI that narrowly. I think defenses would be useful, should they prove feasible, even if offenses dropped to zero, as a hedge against a Soviet treaty breakout or nuclear threats by other countries."

Critics might also argue that although this new walk-in-the-woods agreement might reduce the threat to our ICBM force, it wouldn't eliminate it entirely. The Soviets could still destroy our 1,000 Minuteman silos with as few as 2,000 warheads.

These critics would be right, but there's a simple, long-range solution for the vulnerability of our silo-based missiles: replace them with mobile missiles.

By making our nuclear forces less vulnerable, mobile missiles would accomplish the same goal as SDI defenses. And they would be much cheaper. A force of 500 Midgetman missiles probably would cost $40-to-$50 billion. An SDI system that would allow the same number of silo-based ICBMs to survive a Soviet attack could cost at least twice as much.

The Reagan administration should drop its Geneva proposal to ban mobile missiles and learn to love them. Both sides will be stronger and more secure if they replace their vulnerable ICBMs with mobile missiles like the new Soviet SS-25 and the U.S. Midgetman.

The shape of the arms-control deal of the 1980s is obvious. The challenge for the Soviets is to offer the administration strategic benefits that are equivalent to SDI. Then the challenge for Ronald Reagan will be to say yes.

David Ignatius, an associate editor of The Washington Post, is editor of the Outlook section.