Hi! I wrote about the post-war political history here so use this as background. This mainly revolved around the Liberal Democratic Party (LDP) dominance, but it does mention partly why the opposition failed.

How did Japan's socialist party go from being the largest opposition party in the 50s and 60s to falling apart in the 90s?

“Japan's socialist party” is referring to the Japan Socialist Party (日本社会党 nihon shakaitō) to be referred here as the JSP or the Socialists. There is also the Social Democrats (社会民主党/社民党 shamintō) or JSDP.

In technicality, there is the Constitutional Democratic Party (立憲民主党 rikken minshutō) or CDP. This contemporary party is a mixture of both the JSP and other parties, but mostly the JSP. The CDP previously was the Democratic Party of Japan (DPJ 民主党 minshutō), which was created after 1995 as a large opposition party due to mergers of several parties. (FYI There was another Democratic Party during the post-war, but it was composed of ‘left leaning conservatives’ that merged with the LDP, hence the D in LDP). Then in 2009 the DPJ was elected as the government causing the largest break from LDP governance until 2012. For many reasons such as in-fighting, the DPJ broke up after 2012 and reformed as the CDP. The CDP is one of the larger opposition parties, but there are many other parties of different ideological stripes. So yes, the left-wing of the political spectrum is not very healthy in Japan. The JSDP is actually on its deathbed if it does not retain seats in the next election. See here.

Part of the explanation is historical and theoretical in the electoral system of Japan. This is explained by Duverger’s Law. Essentially depending on the electoral system such as a single member district (SMD) such as the United Kingdom or the United States, the political system ‘converges’ to a two party system. Where there are varying electoral systems, there tends to be multi-party system. Duverger’s Law does not necessarily explain why one party dominates. So Japan had the same electoral system until 1993 where there were multiple member districts. See in the post above for explanation (albeit not a great one). So mathematically the choice of one representative in each district tends to dwindle party choices. Whereas multiple members and proportional elections systems can allow for smaller parties to gain footing. So this explains why there was the rise of other parties. So the more choices voters had, the more constrained and disunited opposition was to the LDP.

So there is a history I mention in the above post of how the LDP maintained power. The government fiscal structure is the first thing. There is a lot of urban to rural fiscal transfers throughout the post-war. Then the development of clientism where money essentially maintains a dependent voter base in the countryside to the LDP by using pork barrel politics. Then the bureaucracy being in cahoots with the LDP protects these clientele networks. So Scheiner (2006) uses these three points to explain opposition failure in the country side where the LDP dominates. The opposition really cannot compete on this so it has entrenched a voter base for the LDP. So the historical advantages the LDP helped maintain a system to keep it in power. This has a number of different side effects that the LDP is a rather fragmented party and the prime minister is a rather weak leader in controlling her party.

So the post-1993 electoral reforms actually switched the multimember districts to single member ones. However, they also added the proportional voting bloc system. So this is a mixed system. From what I understand, this can go either way according to Duverger’s Law so it is an on-going process. Yet there is still a pretty good plurality of parties in present day Japan. I go into the 1990s political saga here for the history of why the LDP lost in 1993 and how it relates to electoral reform.

So to pull it altogether. Duverger’s Law first explains that there was going to be a high likelihood of multiple parties. In 1960 some JSP members were punished for publicly supporting the revision of Anpō (the US-Japan Security Treaty). The Socialists had a hardline of pacifist neutrality. These JSP members left the party and formed the Japan Democratic Socialist Party (JDSP) 民主社会党, Minshu Shakai-tō that was more center leaning. This began the slow decline of a two party system. The leftist movement likely stalled as the material conditions got better in Japan during the so-called Japanese ‘high speed growth era’ of the 1960s. There was also disputes here and there between their constituencies, the unions and other groups. The LDP’s support in the urban areas began decreasing in the 1960s, but this support was scattered into the fragmented opposition. This can be found in the other parties that came about during the time such as the (New) Kōmeito (公明党 Clean Government Party), which is mostly an urban party related to the Sokka Gakkai, a conservative Buddhist organization. Then there was always fluctuation of support for the Communists (共産党 Kyōsantō) who were often hard to deal with. There have been many parties throughout the post-war of all stripes often eating away at LDP, but not going to one opposition party. There are several conservative parties founded by rebel former LDP members through various points after 1970. So a unified and competitive opposition party did not necessarily arise as the LDP became weaker. The Socialists were the largest opposition party throughout the post-war, but were constrained by the growing plurality of parties in addition to the incumbency power of the LDP.

In addition, there is speculation that after the Socialists and the LDP became the government together in 1994, it did thoroughly kill the ‘1955 system’, which supposed to be a two party system. See again here. So essentially the Socialist PM Murayama despite being at the helm with about 60 plus percent of the Lower House, it was sharing power with the LDP. So during Murayama’s reign there was no large opposition, but Murayama was cited as having a hard time governing. See Part 2.