Kellner was loyal to the Kaiser’s regime, but still he welcomed the birth of the German democracy after the war. He became a political organizer for the leading political party, the Social Democratic Party of Germany (SPD). From those first days of the Weimar Republic, he spoke out against the danger of extremists, against Communists and the National Socialists (the Nazis). Kellner would show his opposition at rallies by holding Adolf Hitler’s book, Mein Kampf, above his head. He then shouted to the crowd: “Gutenberg, your printing press has been violated by this evil book.” On more than one occasion Kellner was beaten by the Nazis for expressing his views. 