A leader of the Königsberg Social Democrats, Braun became a member of the national party executive in 1911. Two years later he was elected to the Prussian lower house and, after World War I, to the Weimar constituent assembly (1919) and the Reichstag (parliament). His major political role, however, was realized in the government of Prussia, in which he served from 1920 to 1932—with brief interruptions in 1921 and 1925—as prime minister. His administration provided a strong democratic buttress for the government of the Weimar Republic, and, during the late 1920s and early 1930s, a last outpost for the beleaguered German Social Democrats. In July 1932 he and his ministerial colleagues were evicted from office by the German chancellor, Franz von Papen, but Braun still retained a shadowy advisory position in the Prussian government until he emigrated to Switzerland shortly after the Nazi accession to power in January 1933.