The supermajority core of the US Senate and the failure to join the League of Nations

The failure of the United States to join the League of Nations is often considered to be an outcome of isolationist influence. The supermajority requirement of treaty ratification in the US Senate also is blamed for allowing a minority of isolationists to block the will of the majority that supported the treaty. To determine the cause of the failure, I analyze the Senate debate over the treaty using the concepts of the supermajority core and supermajority winset. Using all 157 votes on the treaty, I estimate senatorial positions and the locations of both the status quo and the treaty on the same metric space. From this analysis, I find that isolationists were not influential enough to block the ratification. Instead, President Wilson’s unwillingness to compromise is found to have played a critical role in the treaty’s defeat. The treaty’s defeat thus was not an indication of the power of isolationism. This study contributes to the growing body of literature that debunks claims about the dominance of isolationism in the interwar period. At the same time, the paper demonstrates how the core and winset concepts can be useful in answering substantive collective choice questions.