While the normative configuration and further development of Union citizenship is clearly assigned to EU organs, the same applies for legislative and judicial organs of the Member States when it comes to nationality. Nevertheless, the European Court of Justice (CJEU) gradually increased requirements vis-à-vis Member State nationality law by recourse to primary law although the normative relationship and dependency between the two statuses, as enshrined in Union constitutional law itself, is the other way round. According to the thesis of this contribution and notwithstanding the Court’s cautionary formula according to which the Member States have to respect or take into account Union law when exercising their sovereign rights, the latest judgment and already visible effects of the Court’s previous jurisprudence in Member State law and administrative procedures can be described as a competence fusion through (Union) citizenship. In sum, the requirements it extracts from Articles 20 and 21 of the Treaty on the Functioning of the European Union (TFEU) call into question the principle of conferral and the significance of the constitutional preconditions for the application of the Charter of Fundamental Rights (CFR) as set forth in Article 51 (1) CFR.