Over the past three decades the prison population in the United States has exploded. At first, the research literature focused on economic explanations of the prison growth, but more recently there has been growing interest in the role of politics on the growth of prison populations. The current study extends this interest in political explanations by examining recent political science research and State social policies as explanations of the growth of imprisonment rates. The main hypothesis was that the effect of partisan politics on imprisonment rates would be conditional on the electoral competition encountered by State representatives in their home districts. Data on annual State-level imprisonment rates for 1977 through 1996 were analyzed, as were data on State characteristics and partisan politics. Variables under examination included measures of Republican legislative and executive strength, electoral competition, class and racial threat, unemployment, and various justice system and control variables. Results of statistical analyses indicate that the influence of Republican State legislative strength on prison admissions is dependent on time and the level of competition in State legislative elections. The findings underscore the need for a more thorough understanding of the relationship between partisan politics in the United States and the use of imprisonment. Tables, figures, footnotes, references, appendixes