This book examines the political significance of labor unions in the United States, addressing a gap in existing research by focusing on how unions influence party allegiance. While acknowledging the well-documented decline of organized labor, the study highlights that unions remain politically relevant for a significant portion of Americans. Drawing on extensive U.S. survey data, the book explores two key dynamics: the link between union affiliation and party support, and the role of public attitudes toward labor unions as a social group. It further argues that these relationships are shaped by individuals’ political knowledge—particularly their awareness that the Democratic Party is generally more supportive of labor than the Republican Party. Ultimately, the project underscores that despite their diminished size, labor unions continue to have meaningful impacts on public opinion, partisanship, and electoral politics.