He was invited to Tokyo by Suematsu Kenchō to stay at the residence of the former Mōri clan to help edit a history of the Meiji Restoration.

[citation needed] Afterwards, he went to work for the Yorozu Morning News, where he began to support social justice causes and pacifism.

[citation needed] In 1903, Sakai established the socialist organization Heiminsha, together with Shūsui Kōtoku and Uchimura Kanzō.

In 1922, he became one of the founding members of the Japan Communist Party and was elected to a seat in the Tokyo City Assembly in 1929.

In June 1932, he was admitted to a hospital after an incident of domestic violence under suspicion of insanity, and died of a cerebral hemorrhage on January 23, 1933.