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 returmed 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.
On 31 August 1920, during the Kuusinen Club Incident, Rahja was murdered by the Petrograd Opposition, a faction within the SKP. This so-called "murder opposition" consisted mainly of students of the Petrograd Red Officer School.
The word "Rahjaism" was used to describe the politics of the brothers, who were usually in conflict with their comrades and attracted many enemies. The Rahjas were involved in smuggling, black-marketing, horse-trading and counterfeiting, for example, and they were accused of corruption, extravagant life and drunkenness in the SKP.