During the Finnish Civil War in early 1918, Tuominen sided with the Finnish Red Guards and edited Kansan lehti, a radical social democratic newspaper in Tampere. He was shortly arrested when the White Guards took the city in April, but he was soon released. After the Red Guards were defeated in May 1918 several Finnish radical social democratic leaders fled to Russia, where they split from the mainstream of Finnish Social Democratic Party and founded the Communist Party of Finland in Petrograd in August–September 1918.