The Microfoundations of Party System Stability in Legislatures

Carol Mershon, Olga Shvetsova


What sustains stability in legislative party systems between elections? This question commands attention given the potential for change highlighted in recent work on legislative party switching. In addressing the question, this article echoes a prominent theme in research on legislatures, parties, and party systems: the importance of the party label. The novelty here is the treatment of the individual legislator’s need for manifest loyalty to the status quo party label as the chief constraint that deters incumbents from switching and underpins stability in legislative party systems. Our theory focuses on the value of stable party affiliations to voters and thus to incumbents as well. We extract testable implications and assess hypotheses against an original cross-national dataset of over 4,300 monthly observations of MP behavior in 116 legislative terms. We find that the temporal proximity to elections deters MPs’ moves. This electoral deterrent acquires particular force under candidate-centered electoral systems