Parliamentary signalling and party factionalism: the Labour Party under Keir Starmer

Abstract This article provides the first systematic and data-driven measure of Labour Party parliamentary factionalism under Keir Starmer by examining parliamentary nominations in the 2025 deputy leadership election. Developing an entirely new, comprehensive dataset of all Labour Members of Parliament (MPs) and then using multivariate and bias-reduced logistic regression, our article reveals how personal attributes, constituency characteristics, organizational affiliations, trade union connections and parliamentary behaviour shaped a three-bloc parliamentary factional structure: a leadership-aligned centre backing Bridget Phillipson, a sceptical soft-left coalition supporting Lucy Powell, and a small socialist left nominating Bell Ribeiro-Addy. Drawing on this, we then advance parliamentary signalling theory as a framework for understanding parliamentary choices in intra-party democracy. Through this we advance a transferable approach for analysing party factionalism in other political parties and parliamentary democracies.