This article takes the European Union's constitution‐making as a case study to examine ‘how’ politicization can emerge through contestation by political actors in response to political opportunities. It advances understanding of the conditions and processes through which politicization emerges by undertaking empirical analysis. The primary data source is an original sample of political actors' claims‐making over European integration issues retrieved from news samples in France, Germany and Britain during the constitution event (2000–05). Main tenets of prominent theories on politicization are unpacked and tested in relation to the evidence from the claims‐making analysis. The findings demonstrate the transformative impact of the French referendum as a specific opportunity: politicization was largely restricted to internal national contestation by French actors; political party competition was the prominent contestation form; and the Socialists mobilized against the constitution by advocating ‘Social Europe’. This transformed the political space by introducing competition over Europe into the party system's core.