The micro-foundations of party unity are still an understudied topic in comparative politics. This article explores the effects of political ambition on party loyalty in Members of the Chamber of Deputies’ (MCDs) claimed votes based on attitudinal data. Ambition theory posits that ambitious politicians should act according to the (s)electorate they are aiming to win for the next election, therefore acting prospectively in their legislative votes. Findings suggest that MCDs seeking office in the national executive branch are more likely to follow the party line and less likely to side with their district or vote inconsistently. While inclusiveness of candidate selection procedures alone does not have an effect, its interaction with differing ambitions does. The results provide us with a first systematic analysis of ambition and its effect on party loyalty in 14 countries by examining the individual MCD more closely.