 After the war he became a political organizer for the Social Democratic Party of Germany, which was the leading political party during the time of the turbulent and short-lived Weimar Republic, Germany's first period of democracy.Kellner and his wife, Pauline, campaigned together as Social Democrats against Adolf Hitler and the Nazis.During World War II, working as a civil servant at a small court house, he wrote a diary to record his observations of the Nazi regime.