 A few local members of a far left SPD group, among them Edwin Hoernle and Albert Schreiner, who later became well-known members of the Spartacus League ("Spartakusbund"), visited the youth group in Zuffenhausen and gave lectures.In 1916, Rau joined the Spartacists as well and became a co-founder of their youth organisation.In accordance with the politics of the Spartacists, in 1917 he joined the left-wing Independent Social Democratic Party of Germany (USPD) and in 1919 the Communist Party of Germany (KPD), which had been founded mainly by members of the Spartacus League.