I've only given this a quick read but this Salon article is important: Exposing Bush's historic abuse of power

According to several former U.S. government officials with extensive knowledge of intelligence operations, Main Core in its current incarnation apparently contains a vast amount of personal data on Americans, including NSA intercepts of bank and credit card transactions and the results of surveillance efforts by the FBI, the CIA and other agencies. One former intelligence official described Main Core as "an emergency internal security database system" designed for use by the military in the event of a national catastrophe, a suspension of the Constitution or the imposition of martial law.

Main Core may be the contemporary incarnation of a government watch list system that was part of a highly classified "Continuity of Government" program created by the Reagan administration to keep the U.S. government functioning in the event of a nuclear attack. Under a 1982 presidential directive, the outbreak of war could trigger the proclamation of martial law nationwide, giving the military the authority to use its domestic database to round up citizens and residents considered to be threats to national security. The emergency measures for domestic security were to be carried out by the Federal Emergency Management Agency (FEMA) and the Army.