In 1998, former members of the JSP and of other parties formed the Democratic Party of Japan, and the SDP became a third-tier opposition party, watching its numbers steadily decline.
Pada 1998, sejumlah bekas anggota PSJ dan LDP membentuk Partai Demokrat Jepang, dan PSD menjadi partai oposisi kelas tiga, sementara jumlah anggota terus menurun.