An institutional safety net? How electoral institutions mediate the fortunes of parties under threat

ABSTRACT How do electoral institutions condition the electoral fortunes of parties under threat? In this article, we examine how Germany’s mixed-member proportional (MMP) system has influenced the vote share of the Sozialdemokratische Partei Deutschlands (SPD), a party under threat which has consistently lost votes over the past two decades. Using the 2013 and 2017 waves of the German Longitudinal Election Study (GLES), we find that voters are likely to engage in “sincere” vote switching on both the district and party ballots and that they are less likely to engage in “strategic” vote switching when they cast their district votes than when they cast their party votes, which protects SPD district candidates. Moreover, voters who stay with the SPD when casting their district vote are also less likely to switch their party votes, which protects the SPD’s overall vote share. We thus conclude that Germany’s MMP electoral system serves as an institutional safety net for the SPD as a party under threat. Our findings have important implications for understanding the ways in which electoral rules shape voting behavior and how different rules can mediate the decline of parties under threat.