"The President of the United States has available certain powers that may be exercised in the event that the nation is threatened by crisis, exigency, or emergency circumstances (other than natural disasters, war, or near-war situations). Such powers may be stated explicitly or implied by the Constitution, assumed by the Chief Executive to be permissible constitutionally, or inferred from or specified by statute. Through legislation, Congress has made a great many delegations of authority in this regard over the past 230 years. [...] The National Emergencies Act (50 U.S.C. §§1601-1651) eliminated or modified some statutory grants of emergency authority, required the President to formally declare the existence of a national emergency and to specify what statutory authority activated by the declaration would be used, and provided Congress a means to countermand the President's declaration and the activated authority being sought. The development of this regulatory statute and subsequent declarations of national emergency are reviewed in this report."
CRS Report for Congress, 98-505
Congressional Research Service: https://crsreports.congress.gov/