The Fourth International Caucus (FIC) is a grouping of members of Solidarity in the USA who support the Fourth International. It originated from a group who was expelled by the Socialist Workers Party in 1985. Some of them formed the group Socialist Action, from which in turn came Socialist Unity. The latter joined with the International Socialists and Workers Power groups to form Solidarity as a regroupment project.
During the 1990s, two organizations fused with Solidarity—the Fourth Internationalist Tendency (a group expelled from the SWP) and Activists for Independent Socialist Politics (a Socialist Action split that had previously worked in Committees of Correspondence). In 2002, members of the Trotskyist League joined Solidarity. Many, if not all, of the members of these predecessor groups joined the FIC.