Social Democratic Party (SDP)
The Social Democratic Party (社会民主党, SDP) was founded in 1996, in a renaming of its predecessor the Japan Socialist Party (日本社会党, JSP), following a series of splits that led former JSP members to form other political parties.
The SDP is against constitutional revision. According to their official website, the SDP defends Article 9, promotes limiting usage of the Self-Defense Forces to exclusively defense-oriented policy (専守防衛) and removal of US bases in Japan, and advocates for social welfare.
As the Abe administration has become more aggressive in urging amendment of the constitution by 2020, they renewed their constitutional stance and published a statement in July 2017, in which they argued that the constitution of Japan is the common property of all mankind. They have also said that Japan pledged to be a peaceful state via the constitution after World War II, and the constitution has helped Japan earn credibility from international society, including neighboring countries in Asia previously invaded by Japan.