Unions began forming in the mid-19th century in response to the social and economic impact of the industrial revolution. National labor unions began to form in the post-Civil War Era. The Knights of Labor emerged as a major force in the late 1880s, but it collapsed because of poor organization, lack of effective leadership, disagreement over goals, and strong opposition from employers and government forces.
The American Federation of Labor, founded in 1886 and led by Samuel Gompers until his death in 1924, proved much more durable. It arose as a loose coalition of various local unions. It helped coordinate and support strikes and eventually became a major player in national politics, usually on the side of the Democrats.
American labor unions benefitted greatly from the New Deal policies of Franklin Delano Roosevelt in the 1930s. The Wagner Act, in particular, legally protected the right of unions to organize. Unions from this point developed increasingly closer ties to the Democratic Party, and are considered a backbone element of the New Deal Coalition.