Aligning with the Comintern's ultra-left Third Period the KPD abruptly turned to viewing the Social Democratic Party of Germany (SPD) as its main adversary. In this period, the KPD referred to the SPD as "social fascists". The term social fascism was introduced to the German Communist Party shortly after the Hamburg Uprising of 1923 and gradually became ever more influential in the party; by 1929 it was being propagated as a theory. The KPD regarded itself as "the only anti-fascist party" in Germany and held that all other parties in the Weimar Republic were "fascist". Nevertheless, it cooperated with the Nazis in the early 1930s in attacking the social democrats, and both sought to destroy the liberal democracy of the Weimar Republic. In the early 1930s the KPD sought to appeal to Nazi voters with nationalist slogans and in 1931 the KPD had united with the Nazis, whom they then referred to as "working people's comrades", in an unsuccessful attempt to bring down the social democrat state government of Prussia by means of a plebiscite.