Objective. Given that the group aspect of party identification forms a central, yet largely unexplored element of American partisanship, social identity theory presents a compelling social-psychological theory of group belonging through which to reinterpret the contemporary understanding of partisanship.

Methods. Using a mail survey of 302 randomly selected Franklin County, Ohio residents, levels of social identification with the Democratic Party, the Republican Party, and political independents are measured using the Identification with a Psychological Group (IDPG) scale. Scores on the IDPG are used to predict attitudes toward parties and the consistency of partisan behavior.

Results. Levels of partisan social identity proved to be significant predictors of political party ratings, ideology, and party activities, even when taking traditional measures of partisan strength into account.

Conclusions. Social identity is a fundamental aspect of partisanship, which, when measured, can lead to superior prediction and understanding of related political attitudes and behaviors.