This paper analyses the influence of national parliaments in the preference formation of member state governments during the Eurozone crisis. Our new data set „EMU Formation“, collected in the framework of the Horizon 2020 project EMUchoices, is based on 86 semi-structured expert interviews with high ranking policy makers that combined closed and open questions, allowing us to assess for each Euro-area member whether the influence of the national parliament, the government majority or the opposition minority changed between 2010-2015. Our analysis indicates that the (Eurosceptic) opposition benefits most in those countries with the strongest parliament participation rights in EU matters. Based on a qualitative analysis of the interviews, it will be shown that there is a significant correlation between an increase in influence of the opposition minority and a high public resonance and contestation of Eurocrisis related policy issues. Surprisingly, the results show that in member states where the opposition was weakened throughout the crisis, the public resonance was higher than in parliaments were the opposition was neither strengthened nor weakened. In the latter case the opposition could not benefit from increased public awareness but was even marginalized. The country reports indicate that a high public resonance despite a weak opposition can be explained in two ways. First as the examples of Germany and Estonia show, it is possible that an opposition emerged outside the parliament that benefits from public resonance and contestation. Second, as the examples of Cyprus and Ireland suggest, if polarization occurs not between domestic actors but between a member state and European institutions or other member states the opposition is not able to benefit from this polarization. Thus we distinguish horizontal and vertical polarization as differently affecting European decision-making.