Membership development after 1945
The General German Workers' Association (Allgemeiner Deutscher Arbeiterverein, ADAV) founded in 1863 and the Social Democratic Workers' Party (Sozialdemokratische Arbeiterpartei Deutschlands, SDAP) founded in 1869 later merged in 1875 under the name Socialist Workers' Party of Germany (Sozialistische Arbeiterpartei Deutschlands, SAPD). From 1878 to 1890, any grouping or meeting that aimed at spreading socialist principles was banned under the Anti-Socialist Laws, but the party still gained support in elections. In 1890, when the ban was lifted and it could again present electoral lists the party adopted its current name. In the years leading up to World War I, the party remained ideologically radical in official principle, although many party officials tended to be moderate in everyday politics. By 1912, the party claimed the most votes of any German party.
Despite the agreement of the Second International to oppose World War I, the Social Democrats voted in favor of war in 1914. In response to this and the Bolshevik Revolution, members of the left-wing and of the far-left of the SPD formed alternative parties, first the Spartacus League, then the Independent Social Democratic Party of Germany (USPD) while the more conservative faction was known as the Majority Social Democratic Party of Germany (MSPD). After 1918, the SPD played an important role in the political system of the Weimar Republic, although it took part in coalition governments only in few years (1918–1921, 1923 and 1928–1930). Adolf Hitler prohibited the party in 1933 under the Enabling Act and party officials were imprisoned, killed or went into exile. In exile, the party used the name Sopade. The Social Democrats had been the only party to vote against the Enabling Act while the Communist Party was blocked from voting.
In 1945, the Allied occupants in the Western zones initially allowed four parties to be established, which led to the Christian Democratic Union, the Free Democratic Party, the Communist Party and the SPD being established. In the Soviet zone of occupation, the Soviets forced the Social Democrats to form a common party with the Communists (Socialist Unity Party of Germany or SED). In the Western zones, the Communist Party was later banned by West Germany's Federal Constitutional Court in 1956. Since 1949, the SPD has been one of the two major parties in the Federal Republic of Germany, with the other being the Christian Democratic Union. From 1969 to 1982 and 1998 to 2005, the Chancellors of Germany were Social Democrats whereas the other years the Chancellors were Christian Democrats. Shortly before the reunification of Germany in 1990, the East German Social Democratic Party merged into the West German SPD.