It has not been much noticed In Britain, but in the US there is a battle royal under way that promises to have a greater long-term effect on our lives than any party conference. This Tuesday brings a serious risk that the Senate will fail to ratify the Comprehensive Test Ban Treaty (CTBT).

The international agreements to limit nuclear weapons testing will be in tatters; the agreed system of verification will implode. The likelihood that nuclear weapons will be used both between countries and in acts of terrorism will be immeasurably increased.

Forty Republican senators are refusing to ratify the treaty which President Clinton signed three years ago, so that it will be simply impossible to muster the votes to pass it in the 100-strong Senate. Worse, attempts to defer the vote to keep the treaty alive are being obstructed by the Republican leadership who suddenly brought the Bill forward in order to kill it. And even if the President and Senate can cobble up a deal to put the vote to one side, the message to India, Pakistan and North Korea, who have not signed the treaty, along with the other 15 states who have signed but are waiting to ratify, is plain. They should develop their nuclear capacity if they can, and test their weapons if they need to. If the world's leading power is reserving its position, why should they hold back?

It is hard to decipher the conservative mind that resists the US ratification. Public opinion is massively in favour of signing. It is in the US interest, you would have thought, to entrench its nuclear dominance by ensuring that there is a world-wide system of surveillance and verification, thus guaranteeing there are no new nuclear weapon states. With ratification, the 44 nuclear-capable countries will have agreed to stop testing atomic weapons. And the country which the Americans consider the most dangerous of all, China, would be frozen at its current capacity of 500 warheads compared to the US's 5,000.

As for guarantees of compliance, there is a global system of 321 sensors Whose installation the signatories agree to accept and which everybody can monitor. Nor is there any need to test nuclear weapons any more to ensure they are still working. The Senate voted to stop testing in 1992, relying instead on the new 'stockpile stewardship programme' under which testing is simulated by computer.

But American conservatives, like their counterparts in Britain, are obsessed with the need to preserve national sovereignty. In the same way Britain's eurosceptics insist that any participation in the euro means signing away sovereignty, even if it Is token sovereignty given the power of today's global financial markets, so American conservatives refuse to sign away US sovereignty over their right to test nuclear weapons. It may be unnecessary and make the world more unsafe - but the US should accept no constraint on its actions. Jesse Helms, leader of the refuseniks, comes from the same philosophic mindset as William Hague and Margaret Thatcher.

Did the deranged Baroness have the non-signing of the Test Ban Treaty in mind when she remarked that all the solutions to world problems have come from the English-speaking peoples? Fifteen seconds with Jesse Helms should disabuse anybody of that notion.

Some of the Republicans have no more noble purpose than to humble President Clinton, so unreasoning is their hatred. The ratification of the CTBT would be a crucial foreign policy success after his failed impeachment, and they choke at the prospect. Others have become obsessed by the security threat to the US allegedly posed by rogue states and terrorist groups who they think could test nuclear devices below the agreed verification threshold, and explode them in US cities while the US had tied its hands by the treaty.

Nuclear terrorism is a real possibility, but will be made more rather than less likely by the non-ratification of the treaty. For example, retired US general Chuck Horner is on record as predicting a nuclear attack on an American city within 10 years - but he does not think the correct approach is to build up the US nuclear arsenal together with regular testing. That will deter no one, certainly not a terrorist organisation smuggling in a nuclear device to hold the US to ransom. Indeed It will make the global capacity to manufacture weapons larger and less regulated, and the risk of terrorism higher.

What the Republican position is doing is to highlight the one-sided nature of the existing treaties on nuclear weapons; they hugely favour the current nuclear weapon states by discriminating against the rest of the world. The big five nuclear powers managed to get the world to sign up to the non-proliferation pact and the last Test Ban Treaty on the promise they would work to eliminate their own dependency on nuclear weapons but, as Senator Alan Cranston told me, the promise was a fraud. Britain and France cling tenaciously to their status; Russia refuses to reduce its nuclear stockpile; China has been spying on the US to improve its own capacity; and the US itself takes care to ensure that it retains its massive overkill, despite its commitments under both Start (Strategic Arms Reduction Treaty) treaties.

The stakes are no longer high enough for all-out nuclear war to be waged.

None of the nuclear weapon states could now wage all-out nuclear war against each other; the stakes are no longer high enough and the irrationality is too great. The era in which the threat of mutually assured destruction paradoxically assured peace is over. Instead, as a growing number of US strategists and military chiefs assert, notably General Lee Butler, former commander-in-chief of US Strategic Air Command, the possession of useless nuclear weapons is a constant invitation to India, Pakistan, Korea, Iraq, Iran and fundamentalist terrorist groups to make sure they have them too. Appeals from nuclear weapon states not to possess the bomb because of the wider needs of humanity are demonstrably mere cant.

Blair, Chirac and Schroder issued a passionate joint appeal to the Republican Senators on Friday but in the Senate debate they replied by saying they did not care about their allies. The time has come for a more dramatic gesture. The British and French should commit to phasing out our nuclear stockpile as part of a deal in which Russia and the US both agree to ratify the Test Ban Treaty and live up to their commitments under the two Start treaties to halve their stocks of nuclear warheads. Then they must proceed to full elimination, the long-term goal that Clinton has asserted is the US aim.

China must agree too. We are deterring nobody at present, and the expense - just of maintaining the weapons and delivery systems - is vast. Nuclear weapons' time is up, and Europe should recognise the new reality by declaring the whole continent a nuclear weapon-free zone.