Until another party do us part? Party members' electoral disloyalty in Portugal

This article investigates whether and why party members sometimes vote for other parties. Although often taken as parties' most loyal voters, recent evidence suggests that members' vote is not unconditional. Using novel survey data from Portugal, we examine rates of defection by members, test the validity of existing explanations and assess the extent to which traditional theories of party membership can account for such behavior. We argue that ideological incongruence, satisfaction with intra-party functioning, incentive structure and party activism play a key role in explaining why a member casts a vote for another party while maintaining a formal connection with their own party. Findings support the impact of internal discontentment and engagement in party activities, as well as instrumental motivations for members of smaller parties. We also explore how motivations relate to strategic considerations in driving defection. Results differ across parties, suggesting the importance of of party models and legacy. They have important implications for party membership and mobilization, as well as for future research on membership decline and the consequences of party cohesion and factionalism.