The American Federation of Labor and Congress of Industrial Organizations (AFL–CIO) is the largest federation of unions in the United States. It is made up of fifty-five national and international unions, together representing more than 12 million active and retired workers. The AFL–CIO engages in substantial political spending and activism, typically in support of Democrats and liberal or progressive policies.
The AFL–CIO was formed in 1955 when the AFL and the CIO merged after a long estrangement. Membership in the union peaked in 1979, when the AFL–CIO had nearly twenty million members. From 1955 until 2005, the AFL–CIO's member unions represented nearly all unionized workers in the United States. Several large unions split away from AFL–CIO and formed the rival Change to Win Federation in 2005, although a number of those unions have since re-affiliated. The largest unions currently in the AFL–CIO are the American Federation of Teachers (AFT) with approximately 1.7 million members and American Federation of State, County and Municipal Employees (AFSCME), with approximately 1.4 million members.