It is common to consider mass politics and Eurosceptic politicization as ‘post-functionalist’ constraints that encourage differentiated European integration. This study argues that the relevance of Euroscepticism depends on who wins the domestic competition for government office. European mass politics are organized as delegation systems. These systems concentrate authority in the government and give little influence to parliaments and publics. If Eurosceptic parties reach the government, they will push for differentiation and even disintegration. If pro-EU parties succeed, uniform integration is likely to prevail. An empirical analysis of differentiated integration from the 1992 Maastricht Treaty to the 2016 Single Resolution Fund shows that only government Euroscepticism – rather than opposition, extra-parliamentary, or popular Euroscepticism – encourages differentiation. This study explains how uniform integration can prevail even in ostensibly Eurosceptic countries. It suggests that the impact of Eurosceptic politicization depends on party competition and is often more limited than might seem at first sight.