He joined the Russian Social Democratic Labour Party in 1903. He was also active in the Finnish labour youth movement. He was active in the 1905 Revolution working as part of the Bolshevik faction in Kronstadt. Following the defeat of the revolution, he fled to Kuopio, Finland and became active in the socialist movement there. In 1913 he returned to St Petersburg, and went on to become active amongst the Petrograd Bolsheviks. After the Bolshevik Coup he was sent to Finland to organise the Red Guards. As the Finnish Civil War begun, Rahja was wounded in the Battle of Kämärä and stayed in the hospital until the end of war in May 1918. Following the defeat, Rahja fled to the Russian SFSR and became a founding member of the Finnish Communist Party (SKP). He was a delegate for the SKP and the First and Second congresses of the Third International.