The cost of compromise: How coalition participation shapes party membership

How does coalition participation influence party membership? We argue that parties that take part in coalition governments are likely to experience less favorable party membership developments than parties governing alone because of the compromises they have to agree on in multi-party cabinets. This should be particularly likely for parties in ideologically diverse coalition governments since parties participating in such cabinets are more likely to have to deviate from originally promised policy positions; and for junior coalition partners, as they have less influence over policy and are less able to claim credit for government achievements. We evaluate these expectations on the basis of a new dataset that covers information on (1) the number of members in all German state parties between 1990 and 2023, (2) the ideological profile of the state parties, and (3) the status of a state party as a member of a state coalition government. We find that junior coalition partners lose significantly more members than parties that govern alone. Furthermore, an increasing programmatic distance within the cabinet on economic issues is related to a decreasing number of members of the respective government parties.