Analyzing the voting behavior of Swiss members of parliament (MP) using newly collected individual, district, and cantonal level data, we show that both electoral disproportionalities and the insurgent parties’ electoral potential are important determinants of MP voting behavior on the adoption of proportional representation (PR). However, in contrast to the prominent electoral threat thesis, the insurgent party’s high electoral potential decreases the probability that MPs of established parties support PR. The reason for this relationship is partisan redistricting, whose relevance has so far been largely ignored in the literature. We demonstrate that adapting electoral district boundaries for political reasons, if possible in a given institutional context, can be a powerful alternative to the adoption of PR, because it allows established parties to retain parliamentary majorities even as an insurgent party’s electoral potential increases.
