The Clinton administration is on a slippery slope toward deploying a ballistic missile defense system that might protect the United States against very limited ballistic missile attack but almost surely would violate the Anti-ballistic Missile (ABM) Treaty. Before heading down this path, the president must explain to the American people the strategic rationale for turning away from a generation of arms control and nonproliferation policy.

The administration took the first step down the slope when Defense Secretary William S. Cohen announced that the projected defense budget through 2005 would include funds for actual deployment -- not just research and development -- of a nationwide missile defense system. A deployment decision is not due until next year, but the Pentagon already is surveying possible sites in Alaska and North Dakota, and President Clinton has informed Russian President Boris Yeltsin that we might decide to develop a system that conflicts with the ABM Treaty.

If that decision is made, the United States would seek to amend the ABM Treaty to permit deployment of our system, while ensuring that it would not be able to thwart a major Russian attack. While Cohen affirmed that "the ABM Treaty is in our overall interest," he and other U.S. officials note that should such negotiations fail, we retain the right to withdraw from the treaty -- with six months' notice -- if our "supreme interests" are jeopardized.

Taken in isolation, these actions might be reasonable responses to a changing world. North Korea's attempted satellite launch last year raises a prospect that, within a few years, new countries could have missiles capable of delivering warheads to the United States. In this sense, the administration must be ready to proceed if a decision to deploy a missile defense is made next year.

From a strategic perspective, however, the administration has yet to make the case. Cohen predicted that "technological readiness will be the primary remaining criterion" in a deployment decision next year. But that omits several important concerns. Here are three basic questions that need to be addressed:

Will it work?

I'm a big believer in American know-how; but after 15 years of missile defense research and development, what have we to show for it? Our most developed theater missile defense system, aimed at missiles slower than intercontinental ballistic missiles (ICBMs), has failed every flight test against a target. We just postponed by two years the earliest deployment date for a national defense system. Is it wise to decide on deployment next year, before any tests of actual hardware? Will a "thin," ground-based system work against ICBM warheads mixed with decoys or other countermeasures? No one knows.

Is this the best way to ensure our security?

Even if a national missile defense becomes technologically feasible, will it be in our national interest to deploy it? Would tens of billions of dollars be better spent on maintaining deterrence through our offensive power, which has kept the nuclear peace for more than 50 years? Could we not persuade North Korea to end its long-range missile programs for a fraction of the likely price of a national missile defense? What factors, other than Republican political pressure and the looming 2000 election, impel a decision to deploy a national missile defense once "technological readiness" is achieved?

How will it affect strategic stability and nonproliferation?

The overriding strategic interest of the United States is to deter others from attacking with weapons of mass destruction. Deterring countries such as Iran and North Korea is surely part of this objective, but so is maintaining our deterrence relationship with Russia -- "strategic stability," in which neither side is tempted in times of crisis to engage in a first strike at the other. Equally important is the need to counter the proliferation of weapons of mass destruction and long-range missiles.

The strategic arms control process, already threatened by the Russian Duma's inaction on the START II Treaty, could collapse because of Russian concern and anger over missile defense. To win Russia's assent on an amended ABM Treaty, the Pentagon may offer to scrap the ban on multi-warhead ICBMs, the capstone of START II. These missiles can overpower missile defense by delivering more warheads, which is why the Pentagon might offer the deal. But they also threaten strategic stability, as they present a lucrative first-strike target in a crisis.

Were strategic arms control to collapse, would the Nuclear Non-Proliferation Treaty also be threatened?

The "grand bargain" of that treaty was that the nuclear-weapon powers would move toward nuclear sarmament in return for other countries' forbearance from acquiring such weapons. Progress in strategic arms control is our sign of good faith in this regard. Would a breakdown in arms control lead other countries to conclude that the limits on them no longer applied?

A "thin" national missile defense may be the best way to deter smaller countries that develop long-range missiles, while maintaining traditional nuclear deterrence with Russia. That is far from clear, however, and the administration has yet to present its strategic rationale. The time to make that case is now, before the slide down that slippery slope becomes irreversible.

The writer, a senator from Delaware, is the senior Democrat on the Foreign Relations Committee.