She married Hermann Duncker, by now an economics student, towards the end of 1898. Their daughter, Hedwig, was born in 1899. In the meantime, Käte immersed herself in the leftwing politics of the city, becoming first a member of the Social Democratic Party ("Sozialdemokratische Partei Deutschlands" / SPD) and then a party worker, although legally it only became permissible for women to participate in politics in 1908. She employed her teaching skills, giving lectures to the Workers' Education League covering literature, pedagogy, history, socio-politics and economics. She became the chair of the "Association for women and girls of the working class" ("Verein für Frauen und Mädchen der Arbeiterklasse") in Leipzig.
