This paper analyzes the politicization of international institutions, that is, a process in which international institutions become salient and controversial on the level of mass politics. Differentiating between dissenting and supportive acts of politicization, the paper matches data on party manifestos from 22 OECD countries to test a number of alternative explanations for varying stances of parties on the internationalization of governance. Results suggest that scholarly debate tends to overestimate the role of globalization for driving politicization, while institutional variables are too often neglected, although they are crucial to understanding the different levels of politicization. The results of the analysis support neofunctionalist expectations, according to which institutions attract societal awareness and demands in terms of “gravity of power.” Nevertheless, a significant amount of dissent seems to be driven by legitimacy concerns. Additionally, I find strong evidence that parties operate differently depending on their ideological positioning across the political spectrum and their size.