Bell joined the Independent Labour Party in 1900, then moved in 1903 to the Marxist Social Democratic Federation. However, within months, he joined with other dissident members to form the Glasgow Socialist Society, soon renamed the Socialist Labour Party (SLP). He became a leading figure in the party, but was expelled in 1907 for arguing that the SLP should not favour the Industrial Workers of the World. He was able to rejoin the following year, convincing the majority of the party to form the Advocates of Industrial Unionism.