Are members of parliament (MPs) whose ideological position is relatively distant from that of their party more likely to switch parties? From a theoretical perspective, the answer is affirmative, in line with models arguing that ambitious MPs switch parties to influence policy according to their ideal positions rather than to maximise their electoral support or maintain the benefits of their office. However, few studies have empirically tested this expectation. The main reason is that accurate measures of individual MPs’ policy positions are difficult to obtain because roll-call votes in parliamentary systems do not allow distinguishing between party cohesion, i.e. preferences’ homogeneity and party discipline. Our study attempts to address these limitations by relying on estimates of individual MPs’ policy positions derived from legislative speeches (independent variable) to examine their switching behaviour. We employ data collected by the authors for the 18th Italian Legislature (2018-2022). Our aim is to understand the dynamics of party switching by using valid and reliable estimates of ideological differences between parties and legislators. In so doing, we aim to provide a more accurate assessment of the link between ideology and inter-party mobility.
