Once again, in what has become a rite of autumn, Congress must decide at the start of the fiscal year whether U.S. economic and military assistance to Pakistan should continue in the face of Pakistan's relentless pursuit of nuclear weapons. U.S. law bars this aid unless the president certifies that he has received "reliable assurances" that Pakistan "will not acquire or develop nuclear weapons."

But after Soviet troops marched into Afghanistan on Dec. 25, 1979, President Zia quickly sized up how anxious the Americans were to begin using Pakistan to funnel weapons to the Afghan mujaheddin -- something Pakistan already had begun doing on its own in its own security interest. He shrewdly reckoned he could have the U.S. aid he wanted and his nuclear weapons program too.

Sure enough, by 1981, when the White House could no longer certify that Pakistan was not pursuing the bomb, Congress agreed to waive this requirement for six years to permit $3 billion in aid to go forward, and later it renewed the waiver until 1991 to allow another $4 billion in aid to proceed.

By 1985, Pakistan had obtained the design of a tested bomb from China and was preparing to enrich uranium to weapons grade. It was putting itself in a position to build a bomb without violating a surviving U.S. prohibition on testing that would trigger a cutoff of aid. Congress responded by requiring the president to certify each year that Pakistan "does not possess a nuclear explosive device" and that the aid "will reduce significantly the risk" that Pakistan will possess one. Congress settled on this standard only after being informed by the White House that aid to Pakistan would have to be terminated if a tougher version approved by the Senate Foreign Relations Committee were enacted.

Each year since the weaker standard, the Pressler amendment, became law, the White House has managed -- with progressively tortured analysis and expressions of increasing concern -- to make the finding that has allowed U.S. assistance to Pakistan to continue. No matter that Zia reneged on a promise to President Reagan to stop producing bomb-grade uranium, or that Pakistan persisted in its illegal buying sprees for weapons components in the United States, or that it built plutonium and tritium production plants with equipment acquired illegally in Europe or that it is now designing aerodynamic bomb casings for nuclear warheads and outfitting its U.S.-supplied F-16s to deliver them.

Now, no surprise, it appears the president finally will be unable to certify that Pakistan has not built a bomb. And, no surprise, Congress is being invited once again to ratchet down the law as Pakistan ratchets up its nuclear weapons program. This time, however, Congress must move U.S. non-proliferation interests to the head of the line of competing priorities to get a handle on a nuclear situation that is fast moving out of control.

The situation is that both India and Pakistan have nuclear weapons, or at least the wherewithal to assemble components into deliverable warheads quickly -- and both sides know it. India still has a decided advantage in numbers and sophistication of warheads, but Pakistan by now is seen by India as having a credible deterrent. South Asia thus becomes the first region of the developing world in which principal rivals have achieved mutual nuclear deterrence.

The challenge for the United States is to act promptly in a way that puts a cap on the present situation, avoids triggering a nuclear arms race and sets the stage for a longer-term process of reduction and elimination of nuclear weapons. The two sides must be persuaded to engage in confidence building and cooperation to make their nuclear programs more "transparent" to one another.

The essential first step is to cut off military and economic assistance to Pakistan -- cleanly, completely, unambiguously. Fortunately, this can be done by the president and Congress doing nothing. No aid can go forward without the required presidential certification. If the president remains silent, and Congress does not act, the next move is Pakistan's. Finally, Pakistan would be given the choice intended by U.S. law: to pursue its weapons program or to continue receiving vitally needed U.S. aid.

Having achieved deterrence, Pakistan could be influenced to accept verifiable constraints on its nuclear program as a condition of U.S. aid being restored. This time, U.S. law should be made explicit, requiring the president to certify that Pakistan (1) is not producing any additional weapons-grade nuclear materials, thus capping its weapons program at enough material for perhaps a dozen devices, (2) is not transferring weapons material, technology or components to other nations; and (3) is actively engaged in confidence building and nuclear arms control efforts in the region.

India, having accepted the reality of Pakistan's deterrent, could yet be influenced to engage in cooperative efforts that will avoid a spiraling nuclear arms race. But India must be convinced that Pakistan has capped its program and is not preparing to test a weapon. That is why the United States must make it clear now that we are removing ourselves as the silent partner in Pakistan's nuclear weapons program.

The writer is president of the Nuclear Control Institute.