WASHINGTON - If the U.S. Senate eventually fails to ratify the Comprehensive Test Ban Treaty, here are some likely scenarios.

India will probably conduct more nuclear weapons tests. Its nuclear scientists and hawkish strategists want a sophisticated arsenal, ranging from small tactical weapons to huge hydrogen bombs. They also wish to overcome doubts about the technical performance of the weapons tested in May 1998.

But ratifying the test ban treaty would tether the nuclear hawks and allow India to concentrate on the economic route to major powerdom.

India's leading statesmen, Prime Minister Atal Bihari Vajpayee and Foreign Minister Jaswant Singh, recognize this and want to avoid a costly and dangerous arms race. A Senate rejection of the test ban treaty would undermine them.

Pakistan would match India test for test. This would lead to the kind of arms race that Presidents Ronald Reagan, George Bush and Bill Clinton have sought to block.

India and Pakistan just battled in Kashmir. The fighting came closer to erupting into an all-out war and possible nuclear escalation than was reported. If more testing occurs, and hawks in both countries are unleashed, defense spending will increase. Pakistan will move closer to bankruptcy. This will heighten the risk of Taleban-like groups gaining power in Pakistan.

China has signed the test ban treaty, but it will not ratify it if the United States doesn't. It assumes that rejection means that Republicans want to conduct more nuclear tests; otherwise, why wouldn't they ratify?

In this case, China will make preparations to resume nuclear testing, especially if India conducts more tests.

The test ban is valuable because it constrains the kind of weaponry advances that the Chinese military might otherwise wish to make.

Japan will face pressure to reconsider its nuclear abstinence if China and India build up nuclear forces.

Test ban opponents in Washington argue that U.S. missile defenses should reassure Japan that it does not need to hedge its bets. But the Japanese, like U.S. allies in Europe, recognize the technical and strategic problems posed by inevitably less-than-perfect defenses.

Senate rejection of the test ban, paired with aggressive promotion of ballistic missile defenses, will prompt China and Russia to feel that the United States is bolstering its capacity for nuclear coercion and possible first use. Moscow and Beijing will augment their nuclear offenses to counter defenses. In this context, Japan (and NATO allies) will feel more threatened. The next U.S. president could then confront a crisis in alliance relations.

Globally, rejection of the test ban will endanger the nuclear nonproliferation regime. In 1995, the international community agreed to extend indefinitely the Nuclear Nonproliferation Treaty on the promise that the nuclear weapon states would complete a test ban treaty by 1996. This was the minimal disarmament condition that the world would accept from the nuclear states. The 187 parties to the nonproliferation treaty will meet next April to review the status of the treaty.

If the Senate rejects the test ban, we can be sure that measures to tighten nonproliferation controls and maintain sanctions on Iraq will be opposed by an outraged international community. Instead of being the champion of nonproliferation, the United States will be seen as the rogue state of proliferation.

Isolationists may say: ''Who needs the nonproliferation regime? If we feel threatened by proliferation, we can take care of it ourselves.'' But the U.S. interest in keeping countries such as Iran from acquiring nuclear weapons requires cooperation from Russia and from European allies in controlling exports. Washington's persuasive powers will be seriously undermined by roguish behavior on the test ban treaty.

Republicans in the Senate who want both to defeat the test ban and to elect a Republican president should be careful. If they reject this treaty, they will create conditions that no new president could welcome.

The United States can ratify the treaty and still legally escape from it if a threat to national security emerges.

The writer, author of ''India's Nuclear Bomb,'' contributed this comment to The Washington Post.