How does the process of party leadership (s)election impact policy positions of political parties? Whilst there has been a great emphasis in the literature on how inter-party competition influences party positions, the literature is scant on the consequences of leadership selection processes on parties’ ability to respond and adapt to the wider inter-party electoral and legislative environment. This is all the more important in a world where voters increasingly feel alienated from established parties and radical populist propositions are becoming prevalent even amongst established parties as they are nowadays. Using Large Language Models to scale ideological positions of individual MPs from legislative speeches and item response theory models to scale positions from elite survey data across different parliamentary systems, this paper analyses how the significance of party membership's influence in selecting the party leader vis-à-vis that of MPs is likely to influence the ideological positions of MPs most likely to run for leadership races within that party, and the positions of the winners of those races. The findings of this paper will have implications to inter-party competitions, legislative coalition building and electoral accountability of parties in parliamentary democracies.