Party people? Measuring interest groups’ proximity to political parties

Research suggests that single-issue groups and labor unions have been incorporated into one of the two US party coalitions (Karol in American gridlock: the sources, character, and impact of political polarization, Cambridge University Press, Cambridge, pp. 68–85, 2015; Lacombe in J Polit, 10.1086/704329, 2019). But political scientists have not examined whether other types of groups, including trade and professional associations, have experienced such incorporation. I argue that some groups are closely aligned with one of the political parties, but others choose not to affiliate for strategic reasons rooted in their membership and policy goals. I develop a new measure—Party Proximity Index—to plot a unique dataset of 2040 interest groups on a partisan scale that provides valuable information about variation in interest group lobbying strategies, the ways interest groups mobilize their members, and groups’ relationships to parties over time.