There is great consensus in specialized research regarding the interaction of different power resources of the trade unions within the framework of labour relations, distinguishing between those of a structural (position in the labour market), associative (membership and representation) and institutional (participation in collective bargaining and social dialogue) nature. This article reviews the most relevant theoretical approaches and empirical evidence regarding these relations, making special reference to the impact of the crisis on the associative resources (structure and evolution of direct membership and the electoral audience) of Spanish trade unionism.