Final answer:
The decline in union membership during the 1920s resulted from a variety of factors including the influx of immigrants, changes in workforce composition, and legal challenges such as the Taft-Hartley Act of 1947 that allowed workers to opt out of unions.
Step-by-step explanation:
During the 1920s, there was a notable decline in union membership in the United States when compared to the World War I years. Several factors contributed to this decrease, including the influx of immigrants who faced language barriers which made it difficult for them to organize into unions. Furthermore, immigrants were often willing to work in challenging conditions due to their dire circumstances, undermining union efforts to improve labor standards. Moreover, the transition of farmers to urban jobs and their self-reliance background made them less likely to join unions. Additionally, many unions at the time excluded African Americans, which limited the appeal and growth of unions.
Union membership was further influenced by changes in the workforce composition, as there was a slight decrease in blue-collar jobs due to advancements in technology and automation, and a corresponding rise in clerical jobs, which were less susceptible to unionization. Legal factors also played a significant role in the decline. For instance, while the National Labor-Management Relations Act of 1935 had once bolstered unionization by guaranteeing workers the right to organize, the post-World War II Taft-Hartley Act of 1947 rendered the legal climate less favorable by allowing states to enable workers to opt out of unions, contributing to a long-term decline in union membership levels.