In this paper, we examine the relationship between supranational integration and domestic party-system polarization (extremism). We first construct a theoretical argument that uncovers the key trade-off between the “output legitimacy” of a supranationally integrated party system and the inevitable loss of “input legitimacy” caused by externally imposed policy constraints. This translates into a strategic tradeoff between responsibility and responsiveness at the party level of electoral competition. We hypothesize that while moderate supranational policy constraints can initially speed-up platform convergence, ever-closer political integration may reverse the trend towards higher levels of party-system polarization and party extremism. We apply our framework to the case of EU integration and test our key non-monotonic prediction both at the party-system level of polarization and at the party level of ideological extremism. Finally, we apply to synthetic control method (SCM) for causal inference in comparative case studies to study how political integration and supranational policy constraints have affected their overall level of party-system polarization over time. Our overall empirical analysis strongly corroborates our theoretical argument.

中文翻译：

---

“收回控制权”？超国家一体化对政党制度两极分化的影响

在本文中，我们研究了超国家一体化与国内政党制度两极化（极端主义）之间的关系。我们首先构建一个理论论证，揭示了在超然整合的政党系统的“输出合法性”与由外部施加的政策约束导致的“输入合法性”不可避免的损失之间的关键权衡。这转化为选举竞争在政党层面上责任与回应之间的战略权衡。我们假设，尽管适度的超国家政策约束可以最初加速平台融合，但日益紧密的政治整合可能会扭转朝着更高水平的政党系统两极分化和政党极端主义的趋势。我们将我们的框架应用于欧盟一体化的案例，并在党派制度的两极分化和意识形态极端主义的党派两方面测试我们的关键非单调预测。最后，在比较案例研究中，我们将综合控制方法（SCM）应用于因果推理，以研究政治整合和超国家政策约束如何随着时间的流逝影响其政党系统两极化的总体水平。我们的整体实证分析强烈证实了我们的理论观点。