The iron triangle is the relationship between Congress, federal agencies and lobbying groups, according to Auburn University’s Paul Johnson, Ph.D. Special interest groups donate money to Congressional leaders to legislate for particular programs, the federal agencies use lobbyists and connections to influence legislation, and Congressional leaders receive agency support for the continuation or implementation of certain bureaucratic policies. This network of Congressional officials, lobbyists and bureaucrats forms the Iron Triangle.
Interest groups provide data for agencies and Congressional members to use in committee to legitimize a particular stance. Interest groups also spend money on personal donations and the use of a contact network and use funds to craft national messaging on an issue or program. The federal agencies lean on these interest groups to press Congressional committee members with expertise in a specific area. With the backing of special interest groups and the support of federal agencies, Congressional committees act as a barrier against executive-level involvement through the rule-making or budgetary process. This limits the balance of power between the president and the rest of the executive branch.
This system is in contrast to the ideal of Weberian bureaucracy that spawned the merit-based system of most governmental hiring. It includes a strict set of rules and conduct from each member of the bureaucracy.