In the 1912 federal election, the SPD won 34.8 percent of votes and became the largest party in the Reichstag, but was still excluded from government.

The two groups merged in 1875 to create the Socialist Workers' Party of Germany [de] (German: Sozialistische Arbeiterpartei Deutschlands).

From 1878 to 1890, the Anti-Socialist Laws banned any group that aimed at spreading socialist principles, but the party still gained support in elections.

[13] In the 1912 federal election, the SPD won 34.8 per cent of votes and became the largest party in the Reichstag with 110 seats, although it was still excluded from government.

[14] Despite the Second International's agreement to oppose militarism,[15] the SPD supported the German war effort and adopted a policy, known as Burgfriedenspolitik, of refraining from calling strikes or criticising the government.

Anti-war members were expelled in 1916 and 1917, leading to the formation of the Independent Social Democratic Party of Germany (USPD).

On 9 November 1918, leading SPD member Friedrich Ebert was designated chancellor and fellow Social Democrat Philipp Scheidemann, on his own authority, proclaimed Germany a republic.

[20] The SPD government, committed to parliamentary liberal democracy, used military force against more radical communist groups, leading to a permanent split between the SPD and the USPD, as well as the Spartacist League which would go on to form the Communist Party of Germany (KPD) and integrate a majority of USPD members as well.

[23] The position of chancellor was held by Social Democrats until the 1920 federal election, when the SPD lost a substantial portion of its support, falling to 22 per cent of votes.

[24] As Germany was struck hard by the Great Depression, and unable to negotiate an effective response to the crisis, Müller resigned in 1930.

The SPD was sidelined as the Nazi Party gained popularity and conservatives dominated the government, assisted by Hindenburg's frequent use of emergency powers.

The Reichsbanner Schwarz-Rot-Gold, the SPD's paramilitary wing, was frequently involved in violent confrontations with the Nazi Sturmabteilung.

[25] The Nazis overtook the SPD as the largest party in July 1932 and Adolf Hitler was appointed chancellor in January 1933.

In the Soviet occupation zone, the SPD was forcibly merged with the KPD in 1946 to form the Socialist Unity Party of Germany (SED).

[28] In West Germany, the SPD became one of two major parties, alongside the Christian Democratic Union (CDU).

After the 1969 federal election, the SPD's Willy Brandt became chancellor in a coalition with the liberal Free Democratic Party.

It underwent a major shift in policies, reflected in the differences between the Heidelberg Program of 1925 which called for "the transformation of the capitalist system of private ownership of the means of production to social ownership"[43] and the Godesberg Program of 1959 which aimed to broaden the party's voter base and to move its political position toward the political centre.

Under the chairmanship of Kurt Schumacher, the SPD was a socialist party representing the interests of the working class and of trade unions.

Led by Kurt Schumacher after World War II, the SPD initially opposed both the social market economy and Konrad Adenauer's drive towards Western integration fiercely; after Schumacher's death, however, it accepted the social market economy and Germany's position in the Western alliance in order to appeal to a broader range of voters.

The metropolitan Ruhr Area, where coal mining and steel production were once the main industries, have provided a significant base for the SPD in the 20th century.

Small town and rural support comes especially from the traditionally Protestant areas of northern Germany and Brandenburg (with previous exceptions such as Western Pomerania where CDU leader Angela Merkel held her constituency, which the SPD gained in 2021) and a number of university towns.

A striking example of the general pattern is the traditionally Catholic Emsland, where the Social Democrats generally gain a low percentage of votes, whereas the Reformed Protestant region of East Frisia directly to the north, with its strong traditional streak of anti-Catholicism, is one of their strongest constituencies.

The social democrats are weakest in the south-eastern states of Bavaria, Saxony and Thuringia, where the party's percentage of votes dropped to single-digit figures in the 2018 and 2019 elections.