Social Democratic Party of Japan (SDPJ) / Shakai Minshuto

The Social Democratic Party of Japan (SDPJ; called the Japan Socialist Party until 1991) was for many years prior to the rise of the Democratic Party of Japan (DPJ) the largest opposition party. It acquired seventy seats in the July 1993 House of Representatives election and joined the Hosokawa coalition. Like the LDP, the Japan Socialist Party resulted from the union of two smaller groups in 1955. The new opposition party had its own factions, although organized according to left-right ideological commitments rather than what it called the "feudal personalism" of the conservative parties.

In the House of Representatives election of 1958, the Japan Socialist Party gained 32.9 percent of the popular vote and 166 out of 467 seats. After that, its percentage of the popular vote and number of seats gradually declined. In the double election of July 1986 for both Diet houses, the party suffered a rout by the LDP under Nakasone: its seats in the lower house fell from 112 to an all-time low of eighty-five and its share of the vote from 19.5 percent to 17.2 percent. But its popular chairwoman, Doi Takako, led it to an impressive showing in the February 1990 general election: 136 seats and 24.4 percent of the vote. Some electoral districts had more than one successful socialist candidate. Doi's decision to put up more than one candidate for each of the 130 districts represented a controversial break with the past because, unlike their LDP counterparts, many Japan Socialist Party candidates did not want to run against each other. But the great majority of the 149 socialist candidates who ran were successful, including seven of eight women.

Doi, a university professor of constitutional law before entering politics, had a tough, straight-talking manner that appealed to voters tired of the evasiveness of other politicians. Many women found her a refreshing alternative to submissive female stereotypes, and in the late 1980s the public at large, in opinion polls, voted her their favorite politician (the runner-up in these surveys was equally tough-talking conservative LDP member Ishihara Shintaro). Doi's popularity, however, was of limited aid to the party. The powerful Shakaishugi Kyokai (Japan Socialist Association), which was supported by a hard-core contingent of the party's 76,000-strong membership, remained committed to doctrinaire Marxism, impeding Doi's efforts to promote what she called perestroika and a more moderate program with greater voter appeal.

In 1983 Doi's predecessor as chairman, Ishibashi Masashi, began the delicate process of moving the party away from its strong opposition to the Self-Defense Forces. While maintaining that these forces were unconstitutional in light of Article 9, he claimed that, because they had been established through legal procedures, they had a "legitimate" status (this phrasing was changed a year later to say that the Self-Defense Forces "exist legally"). Ishibashi also broke past precedent by visiting Washington to talk with United States political leaders.

By the end of the decade, the party had accepted the SelfDefense Forces and the 1960 Japan-United States Treaty of Mutual Cooperation and Security. It advocated strict limitations on military spending (no more than 1 percent of GNP annually), a suspension of joint military exercises with United States forces, and a reaffirmation of the "three nonnuclear principles" (no production, possession, or introduction of nuclear weapons into Japanese territory). Doi expressed support for "balanced ties" with the Democratic People's Republic of Korea (North Korea) and the Republic of Korea (South Korea). In the past, the Japan Socialist Party had favored the Kim Il Sung regime in P'yongyang, and in the early 1990s it still refused to recognize the 1965 normalization of relations between Tokyo and Seoul. In domestic policy, the party demanded the continued protection of agriculture and small business in the face of foreign pressure, abolition of the consumer tax, and an end to the construction and use of nuclear power reactors. As a symbolic gesture to reflect its new moderation, at its April 1990 convention the party dropped its commitment to "socialist revolution" and described its goal as "social democracy": creation of a society in which "all people fairly enjoy the fruits of technological advancement and modern civilization and receive the benefits of social welfare." Delegates also voted Doi a third term as party chairwoman.

Because of the party's self-definition as a class-based party and its symbiotic relationship with Sohyo, the public-sector union confederation, few efforts were made to attract nonunion constituencies. Although some Sohyo unions supported the Japan Communist Party, the Japan Socialist Party remained the representative of Sohyo's political interests until the merger with private-sector unions and the Rengo in 1989. Because of declining union financial support during the 1980s, some Japan Socialist Party Diet members turned to dubious fund-raising methods. One was involved in the Recruit affair. The Japan Socialist Party, like others, sold large blocks of fund-raising party tickets, and the LDP even gave individual Japan Socialist Party Diet members funds from time to time to persuade them to cooperate in passing difficult legislation.

The Social Democratic Party's 2009 manifesto made it clear that it will take part in a new coalition government, thus showing willingness to participation in talks aimed at establishing a coalition government. It put forward the following four basic policies it intends to raise in negotiations for a coalition government: (1) Rebuild the job market and social services; (2) Change economic policy to one of focusing on expanding domestic demand; (3) Correct the unequal tax code that gives the wealthy and large corporations favorable treatment; and (4) Implement the constitutional ideals. In the area of national security, the SDP called for the immediate withdrawal of the SDF from the Indian Ocean and opposes the proposal for the enactment of a permanent law to allow Japan to send its troops abroad without parliamentary approval.

On 14 August 2009, the policy chiefs of the DPJ, the Social Democratic Party and the People's New Party held a press conference to announce the three parties' joint polices for the general election. The joint policies that had been agreed upon in each of the following areas: (1) maintaining the current rate of consumption tax, (2) undertaking a fundamental review of postal privatisation, (3) supporting child-rearing and engaging in both work and family life, (4) enhancing the social security system, including pensions, medical care and nursing care, (5) strengthening employment measures, including radical revision of the legislation covering temporary workers, (6) revitalisation of the regions. These policies were not a manifesto in the event that a coalition government should be formed, but a document stating that these are policies the three parties share and would like to carry out.

Social Democratic Party leader Tadatomo Yoshida acknowledged that the party faced an uphill struggle in the Upper House election 10 July 2016. Yoshida said that he hopes the opposition party will secure 2 seats, and that he will continue to monitor developments. He added that at this point it is too early to say whether he will take responsibility for the election result. Yoshida also said the SDP takes pride in its history as a party that has protected peace and democracy, and that its past achievements have been supported by many people. He expressed readiness to rebuild the SDP.