Résumé : This thesis consists of a comparative study of national parliaments and of the legal and political contestation of European Economic Governance in Portugal and Italy over the period 2011-2020. Using popular sovereignty as our central concept, to be understood as the capacity of peoples and their elected representatives to freely determine policy, we develop an original qualitative research design, using process tracing and filling a significant gap in the literature by comparing developments within parliamentary chambers in Portugal and Italy in an in-depth manner, paying particular attention to national contexts and to specific historical, cultural and ideological characteristics. To that end, we focus on the analysis of constitutions, case law, plenary debates, transcripts of relevant parliamentary commissions, political and programmatic declarations of governments and political parties as well as newspaper and academic articles in Portuguese, Italian, English and French. We also adopt an interdisciplinary approach, drawing insights from other fields such as social psychology whenever relevant as well as from several legal and economic analyses to underpin our arguments. Our findings show there is a significant asymmetry in the structure of legal and political opportunities for contestation across countries and that structural factors, notably the ideological characteristics of national constitutions, play a significant role in determining the success or failure of political and legal contestation of European Economic Governance at the national level. Lastly, we examine the relevance of our findings in the wider European studies literature and conclude that the study of these ideological, cultural and historical features and characteristics are necessary for scholars wishing to achieve a complete and realistic understanding of opposition to European Economic Governance and the varying forms this phenomenon takes in different European countries.