The is a political party in Japan that was established in 1996. Since its reformation and name change in 1996, it has advocated pacifism and defined itself as a social-democratic party. It was previously known as the .
The party was refounded in January 1996 by the majority of legislators of the former Japan Socialist Party, which was largest opposition party in the 1955 System; however, most of the legislators joined the Democratic Party of Japan after that. Five leftist legislators who did not join the SDP formed the New Socialist Party, which lost all its seats in the following elections. The SDP enjoyed a short period of government participation from 1993 to 1994 as part of the Hosokawa Cabinet and later formed a coalition government with the Liberal Democratic Party under 81st Prime Minister Tomiichi Murayama of the JSP from 1994 to January 1996. The SDP was part of ruling coalitions between January and November 1996 (First Hashimoto Cabinet) and from 2009 to 2010 (Hatoyama Cabinet).
In the 2019 Japanese House of Councillors election, the party won four representatives in the National Diet, two in the lower house and two in the upper house. In November 2020, the party entered into a merger agreement with the Constitutional Democratic Party. The party president Mizuho Fukushima held her seat, and the party cleared the minimum two percent voter share to maintain its legal political party status, in the 2022 House of Councillors elections.
History
Before 2000
In 1995, the former Japan Socialist Party (JSP) was in a deep crisis, as it faced criticisms on entering a coalition with its longtime rival LDP, and core policy changes. Aiming at saving the party, the leadership of JSP decided to dissolve the party and to establish a new social democratic party. In January 1996, such a new party, the Social Democratic Party, was established along with the dissolution of JSP. De jure, JSP changed its name to the Social Democratic Party (SDP) as an interim party for forming a new party, and a movement for transforming the SDP into a new social-democratic and liberal party was unsuccessful. Under Murayama's successor Ryūtarō Hashimoto (LDP), the SDP remained part of the ruling coalition. Long before the disappointing result in the 1996 Japanese general election, the party lost the majority of its members of the House of Representatives, mainly to predecessors of the Democratic Party of Japan (DPJ) that was formed in 1996, but also some to the NFP and other opposition parties. After its electoral defeat in the 1996 general election when it lost another 15 of its remaining 30 seats in the lower house, the SDP left the ruling coalition which it had entered as the second largest force in Japanese politics as a minor party.
2000s–2010s
The SDP won six seats in the 2003 Japanese general election, compared with 18 seats in the previous 2000 Japanese general election. Its motives against the Self-Defense Forces have reverted into abolishing it in the long term, returning into its opposition against the force it had applied in the 1950s. Doi had been the leader since 1996, but she resigned in 2003, taking responsibility for the election losses. Mizuho Fukushima was elected as the new party leader in November 2003. In the 2004 Japanese House of Councillors election, the SDP won only two seats, having five seats in the House of Councillors and six seats in the House of Representatives. In 2006, the party unexpectedly gained the governorship of the Shiga Prefecture. In the 2009 Japanese general election, the DPJ made large gains and the SDP maintained its base of 7 seats in the, becoming a junior partner in a new government coalition; however, disagreements over the issue of the Futenma base led to the sacking of Fukushima from the cabinet on 28 May and the SDP subsequently voted to leave the ruling coalition.
As of October 2010, the SDP had six members in the House of Representatives and four members in the House of Councillors. Following the 2012 Japanese general election, the party retained only six seats in the whole of the Diet, two in the House of Representatives and four in the House of Councillors. The count lowered to five seats in 2013. In 2013, the party's headquarters in Nagatacho, where the party's predecessor the JSP had moved in 1964, were demolished. The headquarters moved to a smaller office in Nagatacho.