As the formal powers of the European Parliament have increased with successive treaty changes, its committees' administrations have seen a parallel growth. We argue that such administrative capacity is necessary but not sufficient for acting on formal treaty powers. Administrative capacity has to be combined with political capacity in order to muster policy impact in European Union decision‐making. By differentiating between intra‐institutional administrative and inter‐institutional political capacity, we offer a fine‐grained conceptualization of policy capacity while broadening the theoretical and empirical understanding of the European Parliament's administration as an organizational structure of formal and informal working practices, intra‐institutional coordination and inter‐institutional relations. Based on expert interviews, document analysis and participant observation, the case of the Transatlantic Trade and Investment Partnership illustrates how societal politicization of a specific policy issue triggered the European Parliament to exploit the latent potential of its post‐Lisbon administrative capacity by transforming it into a more readily deployable political capacity.