How does party politicisation of the European Union (EU) influence public opinion formation? I employ different methodological approaches and sources of data to demonstrate that party politicisation of the EU exerts a moderating role on the effect of drivers of EU support – individual predispositions, framing effects, and party cues – on public opinion formation. Chapter 2 discusses the measurement of party politicisation of the EU in this thesis. Chapter 3 uses multilevel regressions with cross-level interaction terms to explore how individual predispositions on national politics influence support towards European integration at different levels of party politicisation of the EU. The main findings suggest that when the EU is highly politicised in national party systems, citizens tend to rely less on national political attitudes that depend on satisfaction and evaluation – satisfaction with the national economy and trust in national institutions – to form their views on the EU when the issue is highly politicised. The effect of attitudes towards immigrants on public formation is not dependent on the level of party politicisation of the EU. Chapter 4 relies on original experimental data and disentangles the moderating role of the two main dimensions of party politicisation of the EU – salience and polarisation – on the influence of framing effects on public opinion formation. The results suggest that individuals are less affected by frames on high-salience EU issues than on low-salience EU issues. The evidence also indicates that the effect of polarisation on public opinion depends on the level of issue salience. Polarisation moderates the effect of frames on public opinion only on low-salience EU issues. Chapter 5 exploits the case of the 2019 European Parliament election as a moment of intense party politicisation of the EU. I estimate the effect of party cues on support towards European integration and uncover the moderating effect of the temporal proximity to the election date. The evidence suggests that the positive effect of the EU position of individuals’ preferred parties on public opinion formation decreased after the election.