What is the largest federation of U.S. labor unions?

The largest federation of U.S. labor unions is the AFL-CIO, which stands for the American Federation of Labor and Congress of Industrial Organizations. This federation represents a diverse array of unions across various industries and serves as a central coordinating body for labor unions in the United States.

The AFL-CIO plays a significant role in advocating for workers' rights, influencing labor laws, and providing support and resources to its member unions. Its size and influence come from the broad coalition it forms with many national and international unions, which together advocate for better working conditions, wages, and job security for workers across the country.

While other options represent important labor organizations, they either focus on specific industries or sectors—such as the Teamsters, which primarily organizes workers in the trucking, freight, and related sectors, or the SEIU (Service Employees International Union), which focuses on service workers—or have a smaller overall membership compared to the vast network of unions that the AFL-CIO encompasses. Therefore, the AFL-CIO stands out as the largest and most influential labor federation in the U.S.