Social Democratic Party
The Social Democratic Party (社会民主党) is technically an active party, but effectively is a historic party.
Prior to 1996, the party was known as the Japan Socialist Party (日本社会党).
In part due to the unpopularity of those coalitions, the JSP experienced a ruinous loss of seats in the Japanese Diet, and has never recovered.
History
The Japan Socialist Party briefly held power in the proto-government under the GHQ occupation.
From 1955 to 1993, the LDP continuously held power. The JSP served as the opposition throughout.
Takako Doi (土井 たか子) became party chairwoman in 1986. She pushed for greater inclusion of women in politics and government. The Madonna Boom of 1989 saw a record-breaking 12 women run for the Diet, all as JSP candidates. 11 of those 12 won their elections, and this was considered a great acheivement of gender equality.
Grand Coalition
In the August 1993 general elections, the LDP lost enough seats for a grand coalition government to form. Morihiro Hosokawa (細川 護煕) led this government for 8 months, following which Tsutomu Hata (羽田 孜) led for 2 months.
Towards the end of that period, the JSP left the coalition, thereby depriving it of a majority. Hata resigned in June when the LDP and the SDP joined to form a new coalition, rather than face a vote of no confidence. The remaining coalition members came together to form the new opposition party, the New Frontier Party.
LDP Coalition
Party chairman Tomiichi Murayama (村山 富市) served as prime minister for 2 years.
The coalition was highly unpopular among leftist voters, leading to a significant loss of seats in the 1995 House of Councillors (参議院) election. Prime Minister Murayama resigned in January 1996, handing power back to the LDP under Ryutaro Hashimoto (橋本 龍太郎). Popularity did not improve with LDP leadership of the government. The party experienced a ruinous loss of seats in the October 1996 general elections.
Reformation
In January 1996, the JSP reorganized as the Social Democratic Party. The next general elections, scheduled for October 20th, saw the SDP reduced to just 15 MPs.
The SDP continued as a minor opposition party for the next two decades. It was briefly a member of the DPJ-led coalition government, until Mizuho Fukushima was removed from the cabinet in May 2010.
On November 14th, 2020, the SDP voted to merge with the Constitutional Democratic Party. Almost all members transferred, with Fukushima alone remaining. The party just meets the minimum 2% vote share to retain the legal status of a political party.