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De Leonism, occasionally known as Marxism-Deleonism, is a form of syndicalist Marxism developed by 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 (specialized trade 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. The only way this differs from some currents in anarcho-syndicalism is that, according to De Leonist thinking, a revolutionary political party is also necessary to fight for the proletariat on the political field.
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 workplaces 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.
The existing government would then be replaced with a government elected from within the Socialist Industrial Unions, and the newly elected socialist government would quickly enact whatever constitutional amendments or other changes in the structure of government needed to bring this about, adjourning "sine die". Workers on the shop floor would elect local shop floor committees needed to continue production, and representatives to local and national councils representing their particular industry.
Workers would also elect representatives to a central congress, called an All-Industrial Congress, which would effectively function as the national government. These representatives would be subject to a recall vote at any time. De Leonism would thus reorganize the national government along industrial lines with representatives elected by industry, not by geographic location.
De Leonism lies outside the Leninist tradition of communism. It predates Leninism as De Leonism's principles developed in the early 1890s with De Leon's assuming leadership of the Socialist Labor Party; Leninism and its vanguard party idea took shape after the 1902 publication of Lenin's What Is to Be Done?. Its adherents are generally opposed to the policies of the former Soviet Union, and those of the People's Republic of China, and other communist states, and do not consider them socialist, but rather state capitalist or following 'bureaucratic state despotism'. The highly decentralized and democratic nature of the proposed De Leonist government is in contrast to the democratic centralism of Marxism–Leninism and what they see as the dictatorial nature of the Soviet Union.
The success of the De Leonist plan depends on achieving majority support among the people both in the workplaces and at the polls, in contrast to the Leninist notion that a small vanguard party should lead the working class to carry out the revolution. De Leonism's stance against reformism means that it is referred to by the label 'impossibilist', along with the Socialist Party of Great Britain.
De Leonist political parties have also been criticized for being allegedly overly dogmatic and sectarian. Despite their rejection of Leninism and vanguardism, De Leonism also lies outside the "democratic socialist" and "social democratic" tradition. Daniel De Leon and other De Leonist writers have issued frequent polemics against democratic socialist movements, especially the Socialist Party of America, and consider them to be reformist or "bourgeois socialist". De Leonists have traditionally refrained from any activity or alliances viewed by them as trying to reform capitalism, though the Socialist Labor Party in De Leon's time was active during strikes and such, such as social justice movements, preferring instead to concentrate solely on the twin tasks of building support for a De Leonist political party and organizing Socialist Industrial Unions.
Political parties 
- Industrial Union Party
- League for Socialist Reconstruction
- New Union Party
- Socialist Labour Party (Canada)
- Socialist Labour Party (UK, 1903)
- Socialist Labor Party of America