The Communist Party (CP) and its allies played a role in the United States labor movement, particularly in the 1930s and 1940s, but never succeeded, with rare exceptions, either in bringing the labor movement around to its agenda or in converting their influence in any particular union into membership gains for the Party. The CP has had only negligible influence in labor since its supporters' defeat in internal union political battles in the aftermath of World War II and the CIO's (CIO) expulsion of unions in which the party held the most influence in 1950. The expelled parties were often raided by stronger unions, and most withered away. Historiography Scholars disagree why the American union movement never formed a major labor party, and why American workers have never embraced socialist parties. German sociologist Werner Sombart emphasized the absence of socialism. Seymour Martin Lipset compared the US and Canada and asked why Canada had more success; Ira Katznelson looked at weaknesses in the working class, and Kim Voss emphasized early union formations. Others argue that a strain of American exceptionalism made U.S. workers resistant to parties that emphasized class struggle. Some say that Americans downplayed political and social agendas for the sake of unity, so that short-term gains and building strong unions came at the cost of a potential labor party. Others say the left lost its power to lead the labor movement by what commenters charged were 'ideological zig-zags'. The Communist role within the labor movement support parts of all of these theses. Factionalism, zig zags and retreats After playing a leading role in the United Automobile Workers's (UAW) victories in Flint against General Motors and Chrysler in 1937, the CP found itself under sharp attack from its opponents within the UAW. Homer Martin, first president of the UAW, sought to drive out all of the left activists within the UAW in order to eliminate any rival contenders for power. Martin brought in Jay Lovestone, former executive secretary of the CP before his expulsion in 1929, as his advisor and installed Lovestone supporters in key positions throughout the union. Martin only succeeded, however, in bringing about his own downfall. After he failed to persuade the UAW Convention in 1937 to give him authority to fire organizers and eliminate local union newspapers, Martin set out to expel his rivals. After firing or transferring a number of CP members who had played