Abstract. The article provides a comprehensive analysis of the role of constitutional principles of law in the process of legitimization and functioning of the European Union. The author emphasizes the problems of practical implementation and application of constitutional principles at the supranational and national levels. It is noted that a condition for the EU's stable development is to ensure its legal homogeneity, which implies that the Member States must comply with the basic principles of the EU's functioning and the provisions of two constitutional acts - the Treaty on European Union (TEU) and the Treaty on the Functioning of the European Union (TFEU).
The article adopts a limited view of constitutionalism, which covers only those principles that regulate (a) the grounds for legitimacy at the level of the European Union and (b) the relationship between the supranational and national levels. These principles are: vesting of powers, subsidiarity, proportionality, institutional independence, and delegated sovereignty.
The analysis of the five constitutional principles leads to the following conclusions: The Treaties on the European Union and the functioning of the European Union are not called a "constitution," but they have all the features of a codified body of constituent legal acts; the European Union is established as a federation, although in all respects it is a confederation, since the source of its legitimacy is the principles of delegation, subsidiarity and proportionality; the only direct source of legitimacy in the EU is the European Parliament, whose powers are still limited in certain areas due to the general EU restrictions on issues such as common taxation.
The EU has a lot of potential power, despite the fact that the principle of delegation of powers aims to limit its powers to an exhaustive list of competences; a more effective and formalized platform for adopting laws in policy areas that intersect supranational and national levels is needed; the current system of providing national parliaments with reasoned opinions on draft legislation is rather limited to achieve a balance between the legitimacy of the Union and national legitimacy for powers that are actually exercised.
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