The relationships between the State and political parties have often been analysed in dual terms. Yet, as Katz and Mair already noticed in their well-known (and criticized) article on the emergence of the cartel party, a clear separation between parties and public institutions has never been completely achieved, in the evolution of liberal democracies. In contrast, while parties act as agencies of institutionalization, public institutions recognize (de jure or de facto) parties as the legitimate actors of political representation. From this perspective, it is worth considering party change as a process intertwined also with institutional change. To date, however, the analysis of such a relationship has been neglected by political scientists, who have privileged explanations of party change based on other factors, whether at systemic or at a micro level. By avoiding a priori assumptions about causality, our main research question is the following: is it possible to identify patterns of co-evolution between State institutions – more specifically, public administration – and party organizations? Building on a new institutional approach to organization theory, the aim of this article is to investigate to what extent the evolution in the size of party organizations and in the size of public administration has followed similar trajectories. Our study focuses on the United Kingdom and Italy, from 1950 to 2010. Our findings confirm that parties' external face expands when public spending and the number of public employees increase, and vice-versa. The same holds for parties' internal face, at least in the Italian case.