Small in stature and possessed of a fiery temper and a thick rural Tōhoku accent, he was a loud-mouthed and charismatic figure nicknamed "Kōzō the Ogre.

[1] Sasaki was born while his parents were on the run for having eloped without permission, so he was not entered into a proper family registry until he was 11 years old and therefore could not enroll in regular school until that time.

Thereafter, he became an activist in the Labor-Farmer Movement and became a leader in instigating contentious tenant-landlord disputes, leading to physical brawls with police and multiple arrests.

[2][1] In 1937, Sasaki was elected a Sendai city council member, but lost his post just one year later when he was arrested as part of the Popular Front Incident.

With Suzuki becoming chairman of the newly reunified JSP, Sasaki became the new factional leader of the left wing of the party, a position he would hold for more than two decades.

Beginning in 1960, Sasaki engaged in a fierce rivalry with the moderate socialist leader Saburō Eda for control of the party as well as its policy platform.

Although the party only lost 4 seats in the Lower House, expectations had been high, and Sasaki's hard-left radicalism was blamed for the poor showing.