Answer:
Several factors contributed to the gains in union membership in the United States during the late 19th century and early 20th century, including:
Industrialization: The growth of manufacturing industries created a large working-class population, many of whom were dissatisfied with their working conditions and pay.
Immigration: The influx of immigrants into the United States during this period provided a source of cheap labor, but also helped to organize unions as workers sought better working conditions and higher wages.
Labor unrest: Strikes and other labor actions, such as the Haymarket Riot of 1886, helped to galvanize support for unions and workers' rights.
Government support: Some local and state governments passed laws to support workers' rights and protect them from employer abuses.
Union leadership: Strong union leadership, such as that provided by Samuel Gompers and the American Federation of Labor (AFL), helped to organize and unify workers across industries and regions.