The article discusses the new stage of development of formal institutions of power, which forms a different structure of activities of the political system. In the article the term “legitimacy” from different points of view, including using three models of M. Weber. Deals with various models and classifications of legitimacy in the political system. In detail the sources of power, their formation and influence on the political system. The issues of interaction of sources of legitimacy and power are very important for the functioning of the political system as a whole. Management of political processes creates a model of political institutionalization that occurs in political systems and the inherent democratization. The nature of informal institutions depends on the contradiction in the sphere of formal institutionalization. Cause damage to legitimate the established rules and norms is the displacement of formal institutions by informal. The problem is rooted in the historical past of a democratic regime due to the interaction of two factors: the heritage of the authoritarian regime of informal practices and the accumulation of political and economic problems of the authoritarian system. The deformation of political system with the influence of the above factors creates the conditions for transformation (informal way) liberal-democratic institutions with the dominance of informal practices.