The emergence–and subsequent fall–of left populism has commanded much scholarly attention throughout the past decade. But left populism’s ideological and discursive orientations in respect to other parties of the left, particularly social-democratic ones, remains under-researched. In order to explore this gap, we begin by analysing the crisis of traditional social democracy as a combination of the transition from Keynesian to neoliberal macroeconomic regimes, as well as from catch-all to cartelized party systems–a process in which the 2008 financial crisis and 2010 turn to austerity in the Eurozone acted as critical junctures. We then conduct a study of Spain between 2014 and 2020. During this period, Podemos (left populist) adjusted its stance vis-à-vis the Spanish Workers’ Socialist Party (PSOE, centre-left) from one of competition for leadership within a left or progressive bloc to one of cooperation as a junior partner in government. Podemos’ orientation toward PSOE became the fulcrum of intra-party disputes once it became clear that an electoral defeat of the centre-left by forces to its left would not take place. These disputes, in turn, reflected strategic and organizational choices made by Podemos leaders early on in the party’s life. We analyse electoral speeches and the theoretical production of Podemos’ three original leaders to showcase the complexity of the Podemos-PSOE relationship, and conclude that the rise of left populist parties should be understood as a symptom of social democracy’s crisis rather than the emergence of a populist “wave” on the radical left and radical right extremes of the political spectrum. We also find that, far from serving as a corrective to the problems of social disintermediation associated with traditional cartel parties, the emergence of populist parties relying on tools of digital democracy has served to highlight their enduring importance.