The Bill to ratify the international convention on chemical weapons, which would outlaw the use, production, transfer and stockpiling of poison gas and other chemical weapons, was submitted for consideration more than four years ago. But it has had to surmount strong objections from arms control sceptics even to come to debate.
If the necessary two-thirds majority for ratification is achieved at the vote on Thursday, the United States will have just got in under the wire.
The treaty has already been ratified by more than 65 states and comes into force next week with or without the US.
If the vote goes against ratification, the US will find itself in the unlikely company of such countries as Iraq, Libya and Iran, which have refused to sign. This is an outcome deplored by President Bill Clinton, Secretary of State Madeleine Albright, and defense secretary William Cohen, all of whom have spent weeks lobbying energetically for ratification.
But the outcome is by no means certain. Although the convention has support from Democrats and Republicans and was signed by a Republican president, George Bush, five years ago in Geneva, there is a hard core of Republicans, led by the chairman of the Senate's influential foreign affairs committee, Jesse Helms, that has been adamantly opposed.
They say the treaty would not halt the development or use of chemical weapons, because key countries are not signatories. They say compliance cannot be verified, and that US chemical manufacturers would be subject to "unconstitutional" searches by international inspectors.
Their principal objection, however, is the obligation on signatories to share information about how to protect themselves from chemical weapons. They say this would jeopardise US national security.
Interviewed on television at the weekend, Ms Albright defended ratification, saying: "People will wonder what is wrong with us" if the US fails to ratify a treaty that has "made in the USA written all over it". "Can you imagine," she asked, "what it would be like for us to be on the same side as Libya and Iraq?"
Supporters of the treaty argue that verification provisions are the tightest ever of any arms control treaty. They see the objections as deriving less from the chemical weapons convention as framed, than from what they see as a visceral scepticism of arms control itself in a section of the Republican party.
As recently as two weeks ago, Mr Helms and his allies were holding out for several dozen amendments which would have effectively emasculated the treaty. Now, after a charm offensive by Ms Albright, which included a visit to Mr Helms's home state of North Carolina, and another to Houston, Texas, where she appeared on a platform with George Bush, Mr Helms has lifted some of his objections.
Mr Helms has also wrung a separate concession. Last week, Mr Clinton announced that two independent agencies - the arms control and disarmament agency and the US information agency (which has responsibility for the Voice of America radio station) - would be brought into the State Department. Mr Helms has urged such a reform to increase congressional control of their activities and reduce bureaucracy.
Despite the last-minute horse-trading, there remain on the table five amendments. Each could scupper the treaty.