In 1996 Clinton signed the United States onto the Comprehensive Test Ban Treaty (CTBT), a landmark international agreement that prohibited all signatory nations from testing nuclear weapons. The following year, he sent the treaty to the Senate for ratification. Conservative Republicans took the lead in defeating the treaty in October 1999 by vote of 48 in favor and 51 against, far short of the two thirds it needed to pass. International reaction to the Senate's action was uniformly negative, and the rejection was a political setback for Clinton, who had lobbied actively for its approval. One scholar blames the failure on: