In his home town of Lübeck, the young Herbert Karl Frahm, later known as Willy Brandt joined the SAPD, against the advice of his mentor Julius Leber. In his autobiography, Brandt wrote: In autumn 1931, Nazis and German nationalists, the SA and the men in steel helmets joined together to form the "Harzburg Front". ... It was just at this time that the left wing of the social democrats split off, as a result of measures connected to organisation and discipline by the party leaders. A few Reichstag assemblymen, a number of active party groups - above all in Saxony - and not least a large proportion of young Socialists followed the people who were calling for the founding of a Socialist Workers' Party.
In 1934 the youth of SAPD took part in the foundation of the International Bureau of Revolutionary Youth Organizations. The congress, which was held in the Netherlands, was broken up by Dutch police. Several SAPD delegates were handed over to German authorities. The congress then re-convened in Lille. Brandt was elected to the Secretariat of the organization, and worked in Sweden for the Bureau.
The SAPD was affiliated to the International Revolutionary Marxist Centre, but broke with the main party of that international, the Independent Labour Party, over the question of the united front and popular front.
During the Second World War some SAPD members emigrated to Great Britain and worked for the party there. Many of those became members of the SPD. Therefore the SAPD was not re-founded anew after the Second World War. Willy Brandt even became leader of the SPD.