[6][7] Meanwhile, the fortunes of the PSII waned in 1934 when the Dutch colonial authorities clamped down on nationalist activities and party leader Tjokroaminoto died.

Established to discuss religious matters, pressure from the PSII and the Indonesian Islamic Party at the 1938 conference resulted in it becoming more political in nature.

This resulted in the establishment of the Mediating Body for Indonesian Political Parties (Bapeppi), but as neither Gerindo nor Paguyuban Pasundan were prepared to join it, it achieved nothing.

Later that same year, Mohammad Husni Thamrin from Parindra took the initiative, and was largely responsible for the formation of the Indonesian Political Federation (GAPI), which brought together all the nationalist inorganizations except the PNI.

It included organisations that had taken a more cooperative stance with the colonial government by agreeing to take seats in the Volksraad quasi-legislature, as well as those that were non-cooperative, such as the PSII.

[15] On 12 December 1942, after the outbreak of War with Japan, GAPI and the executive of the Indonesian People's Council (MRI), an organisation comprising GAPI, the MIAI and the PVPN civil service union released a statement calling for the Indonesian population to cooperate with and obey the Dutch colonial to defend peace and maintain order.

[25][26][27] Haroso's brother, Anwar Tjokroaminoto was the only PSSI member in the Wilopo Cabinet, serving as social affairs minister from April 1952 until he resigned in May 1953.

[29] In the 1971 election it won 2.4 percent of the vote and ten seats, but shortly after was forced to fuse into the United Development Party, ending its existence as a separate political entity.