In 1921 he traveled to Petrograd, where Kuusinen's adherents, supported by the Comintern leadership, successfully challenged Kullervo Manner's supporters at the next party congress. Tuominen was elected to the party's Central Committee and was put in charge of its Finnish bureau. He returned to Finland, where he was arrested on January 26, 1922, and subsequently imprisoned for publishing a proclamation urging Finnish workers to fight on the Soviet side during the Soviet-Finnish conflict over Karelia.