Under the US Constitution, the president does not have any special powers during a time of a national emergency. However, presidents have decided over time that they should be able to have emergency powers, and courts have agreed with them. A multitude of new laws, regulations and executive orders give US President Barack Obama a possibility to become the most powerful president in the history of the United States.
During World War II and prior to that, these emergency powers were largely uncodified and were primarily used during times of war. Yet, after 9/11 attacks in the USA, the office of the president has obtained extraordinary powers that can be used in times of national emergency. Therefore, Obama can use the emergency powers to take authority of literally everything.
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President's emergency powers come from dozens of laws, regulations, court decisions and executive orders. There is not a single document that regulates what kind of powers the president can use, how and under which conditions.
For example, a presidential directive that was issued during the second term of George W. Bush entitled "National Security Presidential Directive/NSPD - 51/Homeland Security Presidential Directive/HSPD - 20" is primarily concerned with the continuity of the federal government in the event of a catastrophic emergency. It goes about any incident, regardless of location, that results in extraordinary levels of mass casualties, damage, or disruption severely affecting the U.S. population, infrastructure, environment, economy, or government functions.
As one can see, the term could apply to all sorts of scenarios, and the president essentially could become a dictator.
The document says:
"The President shall lead the activities of the Federal Government for ensuring constitutional government. In order to advise and assist the President in that function, the Assistant to the President for Homeland Security and Counterterrorism (APHS/CT) is hereby designated as the National Continuity Coordinator. The National Continuity Coordinator, in coordination with the Assistant to the President for National Security Affairs (APNSA), without exercising directive authority, shall coordinate the development and implementation of continuity policy for executive departments and agencies. The Continuity Policy Coordination Committee (CPCC), chaired by a Senior Director from the Homeland Security Council staff, designated by the National Continuity Coordinator, shall be the main day-to-day forum for such policy coordination."
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