In 1869 and 1870, Hasenclever was a representative for the General German Workers' Association ("Allgemeiner Deutscher Arbeiterverein", ADAV) in the "Reichstag" (parliament) of the North German Confederation. From 1871 on, he was the last president of the ADAV, until it merged with the Social Democratic Workers' Party ("Sozialdemokratische Arbeiterpartei", SDAP) to form the Socialist Workers' Party of Germany ("Sozialistische Arbeiterpartei Deutschlands", SAP). From 1874 to 1888 he was again a social democrat representative in the Reichstag of the German Empire that had been formed in 1871: Originally for the ADAV, later for the SAP. Hasenclever was member of the party board there as well.