SPD membership statistics (in thousands) since 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 the Anti-Socialist Laws banned any grouping or meeting that aimed at spreading socialist principles, 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 (1914–1918) the party remained ideologically radical in official principle, although many party officials tended to moderation in everyday politics. In the 1912 German federal election the SPD claimed not only the most votes but also the most Reichstag seats of any German party.
Despite the agreement of the Second International to oppose militarism,
the Social Democrats supported war in 1914. In response to this and to the Bolshevik Revolution of November 1917 in Russia, members of the left-wing and of the far-left of the SPD formed alternative parties, first the Spartacus League (1914–1919), then the Independent Social Democratic Party of Germany (USPD, April 1917–1931) while the more conservative faction became known as the Majority Social Democratic Party of Germany (MSPD, 1917–1922). From 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 banned the SPD in 1933 under the Enabling Act and the National Socialist régime imprisoned, killed or forced into exile SPD party officials. 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 administrations in the Western zones initially allowed the establishment of four parties, which resulted in the (re-)formation of the Christian Democratic Union, the Free Democratic Party, the Communist Party and the SPD. In the Soviet zone of occupation the Soviets forced the Social Democrats to form a common party with the Communists, resulting in the Socialist Unity Party of Germany (SED). In the Western zones, West Germany's Federal Constitutional Court later banned the Communist Party (1956). Since 1949, the SPD has been one of the two major parties in the Federal Republic of Germany, alongside the Christian Democratic Union. From 1969 to 1982 and from 1998 to 2005, the Chancellors of Germany have been Social Democrats whereas in other years the Chancellors were Christian Democrats. Shortly before the reunification of Germany in 1990, the East German Social Democratic Party (founded in 1989) merged with the West German SPD.