The proposal, unlike President Reagan's broader Strategic Defense Initiative, would be built on the ground - no weapons in space. And Mr. Nunn would keep the plan strictly within the bounds of a 1972 treaty that limits the United States and the Soviet Union to one shield each with no more than 100 interceptor missiles.

This limited defensive shield, or "insurance policy," as Martin Anderson, a former policy adviser to President Reagan. has called it, could save millions of lives in the event of an accidental launch, a limited deliberate strike by a small nuclear power or even, depending on the shield's design, from a cheap cruise-missile bomb launched by terrorists.

Strangely, this prudent suggestion has met with howls of pain from some of the very people you'd think would be for it. Cost-conscious defense critics say it would cost too much money - admittedly, $5 billion or more - to build such a limited shield. But the correct figure for comparison is the

loss of American cities and civilians that might be destroyed if we don't takeout this insurance policy.

Moreover, Mr. Nunn's shield would serve other functions. Building even this limited system would be a useful testing ground for research on the S.D.I. program. It would also serve as a hedge in case Moscow decided to abrogate the 1972 antiballistic missile treaty. Many experts say the Soviet Union has already violated the treaty.

Arms control advocates, meanwhile, fear that even a limited shield would put us on a slippery slope. America, they say, won't be able to stop: Building the limited shield would lead to more and more defenses and ultimately the decision to discard the ABM treaty and deploy the "Star Wars" plan.

But why should this be? America has deployed offensive missiles and bombers before, right up to the limits imposed by various treaties. No one suggests that we will break the limits just because we do things the treaties allow. The United States started building some multiwarhead missiles in the 1970's, yet this didn't commit us to build the MX missile later.

Perhaps strangest of all, many strong supporters of a high-tech Star Wars shield oppose the Nunn plan, or at least are uneasy about it. They are afraid that Mr. Nunn's proposal would create a defensive ceiling locking America forever into a defense against very light attacks, but nothing more.

In fact, actual work on the basic building blocks of defense is what is most lacking now in the Reagan Administration's S.D.I. scheme.

Given all the emotion generated by the subject, a rational discussion of the Nunn proposal apart from its feedback effects on Star Wars would be difficult. Of all the groups that are suspicious of a limited shield idea, the

arms control advocates seem to have it about right. Building Senator Nunn's defense now probably would lead to further defenses, as people see that the technology is sound and that the deployment of the defenses poses no threat to world peace.

But that's all based on the argument that there are defenses we could deploy in the near future at a cost the country would be willing to bear. The key point is that it should be possible to take out Senator Nunn's limited insurance policy and agree to disagree later about what steps to take next. Surely both friends and foes of Star Wars will be able to make their case just as well with 100 interceptors sitting out in North Dakota somewhere.

Today, 100 interceptors; tomorrow.... Nobody knows. Congress should pass the Nunn proposal this spring, and let the voters decide in the fall what to do next.