"In vulnerability, there is virtue." That was the notion behind Mutual Assured Destruction (MAD), the policy that practically defined the Cold War. For more than 30 years, it barred our country from developing a defense against ballistic missile attacks.

During that time, however, the threat of attack continued to grow as more and more countries acquired missile technology. Faced with our increasing vulnerability - and their moral obligation to protect Americans from all forms of attack - federal lawmakers officially retired the MAD doctrine five years ago. They adopted a new policy requiring the U.S. "to deploy as soon as is technologically possible an effective National Missile Defense system that is capable of defending the territory of the United States against limited ballistic missile attack."

By year's end, our nation will have in place a system able to defend against such an attack. Interceptor missile batteries will be up and running at Fort Greeley, Alaska, and Vandenberg Air Force Base, Calif., offering rudimentary protection from ballistic missiles such as those deployed by North Korea.

Of course, more needs to be done to enable these missile defenses to catch up with the threat we face. Yet some members of Congress - perhaps pining for the "good old days" of MAD vulnerability - want to delay fielding the operational systems and cut the program's budget.

Opponents of the systems cite a recent "technical" report from the Union of Concerned Scientists (UCS) arguing the systems haven't been tested adequately. The report complains the (successful) tests to date weren't fully realistic, that they failed to incorporate all the real-world challenges an operational system must face. But, as Lt. Gen. Ronald Kadish, director of the Missile Defense Agency, rightly observes: "You can't operationally test a system until you put it in place."

Indeed, a primary purpose of fielding the system is to test the technologies. For the last three years, Gen. Kadish's agency has followed a new testing approach - called spiral development - that allows testing capability and operational capability to feed and inform each other. Imposing traditional "linear" testing standards at this stage - as urged by UCS and embodied in legislation that may be offered by Sen. Jack Reed, Rhode Island Democrat - would completely disrupt progress and, paradoxically, produce an even less rigorous testing.

The ultimate aim of the UCS report is change policy rather than to uphold or advance science. Their report presents technical arguments only to support the policy argument that the U.S. is better off remaining vulnerable to missile attack.

The report's failure to make even one recommendation on how to overcome an alleged technical shortcoming or build a more effective missile defense system exposes the true purpose of the authors.

Other members of Congress are prepared to go after funding for the program, arguing that the Pentagon spends too much on missile defense. Sen. Ben Nelson, Nebraska Democrat, may offer an amendment that would cut funding for the construction of additional missile defense interceptors.

But missile defense spending has been - and is projected to remain - at levels that can be considered only modest at best, by congressional standards. The Bush administration's request for the defense authorization bill now before Congress - roughly $422 billion - would devote less than 3 percent of that amount (about $10 billion) to missile defense.

No one is arguing the administration's missile-defense program is perfect. For example, there are more promising approaches to building a Kinetic Energy Interceptor (KEI) system than the one currently on the Pentagon's front burner.

|The administration also should consider trying to adapt the Navy's Standard Missile 2 to missile-defense purposes. These changes could actually accelerate deployment of a fully operational missile defense, but they shouldn't be used as excuses to cut the overall missile-defense budget.

 For the short term, America must continue to test and field defenses able to defeat limited missile attacks on our homeland. With an operational capability only months away, Congress shouldn't change either the law or existing policy. Instead, it should fully fund the administration's missile defense program.

Baker Spring is the F.M. Kirby research fellow in national security policy at The Heritage Foundation.