A theory on party discipline and vote switching by legislators

We develop a new theory of vote switching by legislators that defect from party line voting. In our model, ideological candidates select their party label and, once elected, vote in parliament. Political parties select their policy platform as to maximize votes while preserving their candidates' loyalty. We find that legislators that defect from party line voting arise within the majority party. Besides, the more legislators care about their ideology and the less about their own reputation, the more they defect from party line voting. Interestingly, when a party has larger electoral advantage and its candidates sufficiently care about their reputation, such a party can propose a more polarized policy to avoid party defectors. Our model suggests that polarization incentivizes loyalty. Finally, we discuss the robustness of our results in multi-party systems and multi-constituency systems.