The article considers an analysis of the practice of refusing to register candidates for recognition of their involvement in the activities of organizations recognized as extremist. Attention is drawn to the fact that in many cases, not only official documents, but also materials from social networks were recognized as evidence of involvement. Great importance is given to the justification of the constitutionality of the investigated grounds for refusal of registration, which was reflected in the decisions of the Constitutional Court of the Russian Federation, to the disclosure of the concept of «involvement in the activities of an extremist public association». The author proposes in the future to more clearly formulate possible ties with an organization whose activities were recognized as extremist, the establishment of which may be the basis for denial of registration as a candidate.