By every measure, politics in Washington became increasingly polarized along partisan and ideological lines between the Nixon and George W. Bush (2001) administrations. One component of this development has been the growing partisan disparity in congressional support for presidential initiatives. This trend has a manifest electoral and popular basis. Survey and aggregate electoral data show that the extent to which presidents and members of the opposition party in Congress share electoral constituencies has declined significantly, and the parties' respective electoral bases have become increasingly polarized in terms of both ideology and presidential voting patterns. Presidential job approval data also show a widening gap in the average approval ratings expressed by self-identified Republicans and Democrats. These changes have substantially reduced incentives for opposition members to support the president, while making it politically more attractive for the president's own partisans to support his positions. If, as the evidence suggests, partisan differences in presidential support have sturdy electoral roots, they are likely to persist. The overwhelming bipartisan popular and congressional support initially enjoyed by George W. Bush after the terrorist attacks of September 11, 2001 was unsustainable because it was not supported by a fundamental and durable change in mass political attitudes.