The party was established in December 1948 by members of the Socialist Party Orthodox Faction Diet Members' Group, which was formed by 16 National Diet members who had resigned or been expelled from the Japan Socialist Party after voting against the budget proposed by the Democratic Party–Socialist government due to their opposition to an increase in public transport fares.Haruhiro Fukui (1985) Political parties of Asia and the Pacific, Greenwood Press, pp551–552 It was particularly well-supported in Okayama and Hokkaido where party president Hisao Kuroda and Haruo Okada were based, and was supported by some members of the National Railwaymen's Union and National Communications Workers' Union.
