The Swedish Social Democratic Labor Party (SAP) is widely recognized as the most successful social democratic party in world history. According to conventional wisdom, the SAP laid the foundation for its success in the late 1920s and early 1930s by abandoning Marxist class struggle ideology to become a “people’s party.” This dissertation offers an alternative account of the rise of Swedish social democracy. Drawing on archival research at the Swedish Labor Movement Archives, I trace the party’s ideological development from the 1880s to the 1980s. Through analyses of books, newspapers, theoretical journals, pamphlets, party programs, party executive meetings, party congress debates, and autobiographies, I demonstrate a persistence of Marxist themes well beyond the 1930s. Marxism provided the Swedish social democrats with a critique of capitalism and a political strategy for overcoming it. While Marx’s name has often been associated with revolutionary politics, the Swedish social democrats found in Marx a justification of a reformist road to socialism. Marx and Engels had identified the working class as the main protagonist of socialism, and this was reflected in the orientation of Swedish social democrats, who also appealed to and organized the country’s growing class of industrial laborers. However, the focus on the working class was not exclusive. Swedish social democrats simultaneously appealed to and saw themselves as representatives of the whole people. The Swedish social democrats also drew on Marxism for their vision of a socialist society. Early on socialism was identified with state ownership of the means of production. During the interwar era, however, many Swedish social democrats began to doubt the aptness of state socialization as a remedy for the country’s economic disorders. Socialization was not abandoned wholesale, but themes of planning and control became increasingly central in social democratic ideology. In the postwar period, socialization was demoted to a last resort. When ownership was repoliticized in the 1970s, the SAP acted defensively. Ultimately, the party opted for a neoliberal strategy, shedding itself of its Marxist inheritance.