The cliff-hanging final vote was expected to approve ratification of the international convention on chemical weapons by the two-thirds majority needed, if by the narrowest of margins.
Voting came at the end of a ferocious two-day debate which had been punctuated by much public and private arm-twisting by President Clinton and his supporters. As late as yesterday morning, Mr Clinton sent a letter to the leader of the Republican majority in the Senate, Trent Lott, promising to withdraw the US from the treaty if it resulted, as some of its opponents predicted, in the proliferation of chemical weapons.
That letter swung Mr Lott's potentially casting vote in the President's favour. The previous day, Mr Clinton had gathered together the military top brass and senior politicians, in front of a forest of flags, to hear a series of authoritative defences of the treaty.
Yesterday morning, the Senate met for two hours in a rare closed-door session to hear intelligence assessments. The convention comes into force on 29 April, regardless of whether the United States joins because more than the necessary 65 states worldwide have already ratified it. Advocates of the treaty argued that the US would be seen to be aligning itself with countries such as Libya and Iraq if ratification failed to pass the Senate. They also argued that a ban would make it less likely that US troops would face chemical weapons in combat.
The US signed the treaty five years ago, but it was only 10 days ago that the Senate debate was scheduled, after the chairman of the Foreign Affairs Committee, Jesse Helms - a Republican who led the anti-treaty faction - dropped some of his objections.
Senior members of the US administration, including the Secretary of State, Madeleine Albright, had stressed that ratification of the treaty had become a test of Mr Clinton's ability to steer foreign policy and, as such, a test of his authority as President.