Few issues have proven more contentious in American politics during recent decades than immigration. Although there is wide agreement that the existing immigration system is "broken," major policy reform poses enormous political perils for presidents and congressional leaders. The internal conflicts within the Republican and Democratic Parties are at least as great as those between rival partisans. The obstacles to reform are especially daunting when focused on illegal immigration and the status of unauthorized immigrants. This article explains why immigration policymaking is so politically divisive, and how illegal immigration in particular makes even basic problem definition difficult, focusing on the role of rival ideas and interests, elusive coalition-building, widespread cynicism over past implementation failures, an expanded scope of conflict, and flawed policy alternatives. It also illuminates how the Clinton and Bush administrations formulated strategic responses to pressures for immigration reform. The implications of recent immigration reform politics are striking: Whereas Clinton translated its lethargy and defensive opportunism on immigration policy into consequential electoral gains for the Democrats among Latinos and Asians, Bush's vigorous advocacy produced a rebellious and unmanageable party base and further eroded support for the Republican Party among new immigrant voters and kindred ethnics. Significantly, the Obama administration is almost certain to have far fewer degrees of freedom on immigration than its recent predecessors.