De Leonism, occasionally known as Marxism–De Leonism, is a form of syndicalist Marxism developed by the Venezuelan-American activist Daniel De Leon. De Leon was an early leader of the first United States socialist political party, the Socialist Labor Party of America. De Leon combined the rising theories of syndicalism in his time with orthodox Marxism. According to De Leonist theory, militant industrial unions are the vehicle of class struggle. Industrial Unions serving the interests of the proletariat (working class) will bring about the change needed to establish a socialist system. While sharing some characteristics of anarcho-syndicalism (the management of workplaces through unions), and with the SLP being a member of the predominantly anarcho-syndicalist Industrial Workers of the World (IWW), De Leonism actually differs from it in that he and the modern SLP still believe in the necessity of a central government to coordinate production, as well as in the use of a revolutionary political party in addition to union action to achieve its goals.
According to the De Leonist theory, workers would simultaneously form Socialist Industrial Unions in the workplaces, and a socialist political party which would organize in the political realm. Upon achieving sufficient support for a victory at the polls, the political party would be voted into office, giving the De Leonist program a mandate from the people. It is assumed that at that point, the Socialist Industrial Unions will have attained sufficient strength in the for workers there to take control of the means of production.
The De Leonist victory at the polls would be accompanied by a transfer of control of the factories, mines, farms and other means of production to workers councils organized within the industrial unions. De Leonists distinguish this event from the general strike to take control of the workplaces advocated by anarcho-syndicalists, and refer to it instead as a general lockout of the ruling class.