Several studies to date have argued that politics in Western democracies have become gradually populist since the early 1990s, not only due to the increasing number of populist political actors but also to their mainstream competitors, who have followed suit. The empirical evidence concerning the populist contagion has remained inconclusive, however, and the burgeoning literature has so far focused predominantly on electoral performance as the focus of inquiry, political parties as the unit of analysis, election manifestos as the data source, and a priori identification of cases.This study sets out to revisit the theoretical arguments on populist contagion beyond the party-political sphere while questioning the alleged linear (continuously growing) and one-directional (from populist towards non-populist) pattern of contagion. Instead of portraying the alleged populists as the doer and the non-populists as the done-to and postulating a linear increase of populism, it asserts that the complex processes, which eventually lead to more populism in the parliament, might be reciprocal or even symbiotic.The study takes on the ideational conceptual approach and depicts populism as a thin-centered ideology with two fundamental constitutive components: people-centrism and anti-elitism. Following a case study design and using the textual data from the Prime Minister’s Questions sessions, it measures the level of populist discourse at individual and aggregate levels. Doing so, the study generates a novel data-set that provides insights into the establishment or consolidation of populist sentiments among the members of parliament and reveals the temporal trends in the House of Commons for over a twenty-year period.