Salim became one of the most vocal advocates of the growing Indonesian nationalist movement, in the period known as the
National Awakening. He became a prominent leader in
Sarekat Islam and was considered the right-hand man of its leader,
Oemar Said Tjokroaminoto. The SI contained many political strains, including communists, business leaders, and Islamically-minded reformers. Having spent time in the
Hejaz, Salim was considered to be on the reformist side of the SI and was at odds with the increasingly leftist faction of SI led by
Semaun,
Tan Malaka, and
Darsono; who would eventually break with SI to found the
Communist Union of the Indies. Salim was occasionally accused by those leftists of being too close to the Dutch colonial government. First, his newspaper
Neratja had been funded in 1917 with government money (under the direction of Governor-General
Johan Paul van Limburg Stirum) who wanted a Malay-language forum for perspectives sympathetic to the
Dutch Ethical Policy, However, by 1918
Neratja became a harsh critic of the colonial government, regularly printing reports of mistreatment of Muslims in remote regions of the Indies. And when Salim's
Neratja was the only
Malay language newspaper to applaud his expulsion in 1918, its editors were condemned by other newspapers and forced to apologize. Salim responded that he needed money to continue his activities and that everyone profited from the government in some way or other. Later, after the breakup of the Sarekat Islam, Salim (along with Tjokroaminoto) co-founded the
Islamic Union Party (
Partai Serikat Islam, PSI), which later became the PSII. After Tjokro died in 1934, Salim became the party's intellectual leader. In 1921, Salim was appointed to the
Volksraad, the Indies' mainly symbolic and advisory quasi-legislature, representing Sarekat Islam. Despite being a proficient speaker of the Dutch language, he insisted on delivering his speeches in
Malay, the first member of the council to do so despite hostile reception from other European members. He left the chamber in 1924, frustrated by the chamber's powerlessness (its main function was solely to advise the
Governor-General), and derided it as a mere
komedi omong (talking comedy). Being an outspoken advocate of social change within the Indonesian Islamic community, Salim was widely known for his unorthodoxy on social issues. At the 1927 convention of the national Islamic organization
Jong Islamieten Bond (JIB) in
Surakarta, Salim ripped apart the curtained divider between men's and women's seating area and proceeded to deliver his speech titled ("On Veiling and Separation of Women"). Another area of activism that Salim was active in was workers' welfare. At the request of the prominent Dutch trade union
Nederlands Verbond van Vakverenigingen (NVV), he served as an advisor for its delegation to the 1929 and 1930 International Labour Conventions, both held in
Geneva,
Switzerland. It was alleged that throughout his early years returning from Jeddah, Salim had been in contact with or possibly working for the
Politieke Inlichtingen Dienst (PID), the Dutch East Indies's principal state security and intelligence service. Allan Akbar, a historian, conceded that Salim had been asked for a favour by Datuak Tumangguang, a prominent Minangkabau advisor to the colonial government, to covertly enter Sarekat Islam, especially to investigate the relationship between Tjokroaminoto and the Germans. For six weeks, Salim investigated the Sarekat from within, but in a strange twist of history, decided to join the organization of his own will. == Japanese occupation ==