We develop a theory of foreign policy voting in the Senate premised on the assumption that foreign policy programs are a mostly collective good whose costs and benefits accrue to all, but which generate insufficient political benefits to make such policies politically popular, and often make it difficult for senators to support presidents. While certainly at times in U.S. history, presidents have enjoyed remarkable levels of such support (e.g., during the early years of the Cold War), more often than not, politics do not stop at the water’s edge. Why should this be so? We seek to provide scholars a more comprehensive understanding regarding the underlying proclivity of legislators to support the President given the public goods costs of foreign and defense policy and based upon the issue at stake, the political environment and common partisan and ideological preferences. After elaborating upon this account of foreign policy voting, we test our hypotheses by examining the voting behavior of individual senators on all foreign policy related roll call votes in the U.S. Senate from 1979–2000 on which the President clearly expressed a preference, and find strong support for our expectations.