President Clinton will be lobbying hard this week to win enough votes for the US to ratify the treaty outlawing nuclear testing, but his Republican opponents expect him to fail.
The Comprehensive Test Ban Treaty is seen as essential to halting the nuclear arms race, and a failure by the US to ratify it would be a serious setback and encourage countries such as India and Pakistan to develop their own nuclear weapons, the President has said.
Ratification by the US Senate will require a two-thirds majority, or 67 votes, but President Clinton is sure of only the votes of the 45 Democratic senators and needs to win over 22 Republicans by early next week.
Senior Republican senators, including Mr Jesse Helms, Mr Trent Lott and Mr John Warner, are warning that the treaty will endanger US security and that the CIA has admitted it cannot detect low-level Russian nuclear tests.
But Mr Clinton has pointed out that such tests are hard to pick up now and that if the treaty is ratified there will be monitoring sites set up in Russia to detect any unlawful testing.
Mr Clinton signed the treaty in 1996, but the Republican majority in the Senate made no move to have it ratified until it suddenly announced it would have a debate this week, catching the Clinton administration by surprise.
The treaty has been signed by 154 countries, but will only enter into effect when ratified by the 44 countries which are believed to have a nuclear capability. So far only 23 of these countries have ratified it, and France and Britain are the only ones of the seven declared nuclear powers to have done so.
The White House is planning an all-out lobbying campaign this week to win over Republican senators. Former chiefs-of-staff, Nobel laureates, former cabinet members, private organisations and celebrities are being mobilised to speak out in favour of ratification.
Senator Helms, who heads the influential Senate Foreign Relations Committee which will hold hearings this week, has strongly opposed the treaty. He has said that it would leave the US dangerously exposed and "the American people will be left with no defensive or offensive deterrent. It's not going to happen on my watch."
Senator Warner, who chairs the Armed Forces Committee, is also opposed and says: "I feel an honest analysis of the facts . . . will lead the Senate to a vote of No. This is a very significant vote for each senator."
But Mr Clinton says: "This is very important to protect our people from the danger of nuclear war. It would be in my judgment a grave mistake not to ratify the treaty."
The US has not carried out nuclear tests since 1992 and relies on computer simulation to ensure its nuclear weapons are ready for use. US opponents of the treaty say there is no assurance that Russian low-level tests, which are hard to distinguish from earthquakes, are not taking place underground.