The ISO originated in 1976 among a number of groups in the American International Socialists (IS) that were growing increasingly critical of the organization's leadership. Among them was the self-identified Left Faction, which was led by Cal and Barbara Winslow and supported by the IS's Canadian and British members. The Left Faction and its international supporters maintained that the IS's leadership had acquired a top-down style of operating that depoliticized the organization and placed too much emphasis on sending student activists into working class employment (a tactic referred to as "industrialization"). These disputes followed the disagreements over the 1974 revolution in Portugal. Additionally, the main part of IS thought that there should be attention to rank and file or reform caucuses in unions, whereas the Left Faction contended that in addition to rank and file work, agitation at the workplace for socialism should continue. In 1977, the Left Faction was expelled from the IS and immediately formed the International Socialist Organization. The ISO began publication of its paper, "Socialist Worker", shortly after its formation and produced a monthly print version as well as a daily updated website until 2019. The ISO was initially the U.S. section of the International Socialist Tendency (IST), and followed closely the positions of the British Socialist Workers Party.