Tadokoro was a disciple of Hitoshi Yamakawa, a socialist.
Tadokoro organized with Kyuichi Tokuda, later a Communist leader, the Marxism Research Society. He then set up with the Yamakawa group of Zen-eisha (Advance Guard Society) which established a Marxist guiding principle for the proletarian movement to distinguish it from anarchism.
Teruaki was arrested in the first mass arrest of Communists in 1923 and imprisoned for two years. After release, he started and edited the magazine Kensetsusha (The Builder) (later renamed Youth Movement) and the newspaper Musansha Shimbun.
Tadokoro fought against Fukumotoism. In 1926, he participated in the Japan Labor-Farmer Party organized by the group of Hisashi Aso. When proletarian parties were amalgamated into the Social Mass Party in 1932, he became one of its top leaders.