Kakushin Sētō(革新政党) means reformist party or progressive party in English. Kakushin Sētō is generally 'anti-conservative', and in Japan, it has generally referred to democratic socialist, social democratic and socially progressive parties that respect parliamentary democracy. Japan's "progressive parties" are basically opposed to constitutional amendments led by right-wing conservatives, so they are partly in solidarity with "liberal parties".
1926: During this period, as the proletarian movement was activated, the legitimate socialist party Labour-Farmer Party was founded.
1926: At that time, moderate socialists who were compliant with the system formed the Social Democratic Party. At that time, it showed the relatively most conservative tendency among Japan's three major proletarian parties.
1928: The Labour-Farmer Party was forcibly disbanded as the government launched a massive crackdown on socialist forces.
1932: Moderate leftists supporting the constitutional rule of the Japanese Empire founded Shakai Taishūtō.
1940: With the establishment of a one-party system led by the Shōwa statist Imperial Rule Assistance Association (IRAA), Shakai Taishūtō was absorbed into IRAA, and all other Kakushin Sētō were banned.
2017: When the centrist/liberal Democratic Party attempted to be reasonable with the constitutionalist (supporting the constitutional amendment) right-wing party Kibō no Tō, led by Yuriko Koike, progressive-liberals and constitutionalists in the party against it create the liberal-leaning Constitutional Democratic Party of Japan.
2020: The liberal-leaning "Constitutional Democratic Party of Japan" (old CDP) of Japan was reasonable with the centrist/liberal-conservative "Democratic Party for the People", and the new party's name was "Constitutional Democratic Party of Japan" (new CDP)
^Kosaka, ed. (2013). Social Stratification In Japan. Routledge. p. 128. ... In the 1955 survey, supporters of the conservative party were found to be fairly clearly differentiated from reformist party supporters. With regard to age, older generations tend to support the conservative party whereas the young tend ...
^Kosaka, ed. (1970). Japan Socialist Review - Issues 194-205. p. 199. ... has consistently defended the economic and social interests of the working people through its parliamentary and exter-parliamentary activities, and has won the greater results through its activities than any other progressive party. ...