Q: What is a national emergency?

A: In 1976, Congress passed the National Emergencies Act, which permits the president to pronounce a national emergency when he considers it appropriate. The act offers no specific definition of "emergency" and allows a president to declare one entirely at his or her discretion.

By declaring a national emergency, the president avails himself of dozens of specialized laws. Some of these powers have funds the president otherwise could not access.

Under existing law, emergency powers lapse within a year unless the president renews them. A national emergency can be redeclared indefinitely and, in practice, that is done frequently. There have been 58 pronounced under the National Emergencies Act, of which 31 are still in effect.

Q: When have they been declared in the past?

A: Presidents have declared national emergencies since World War II. President Bill Clinton declared emergencies 17 times, George W. Bush 12 and Barack Obama 13.

The vast majority have been economic sanctions against foreign actors whose activities pose a national threat, according to Elizabeth Goitein, co-director of the Brennan Center for Justice's Liberty and National Security Program. A handful of others have involved noneconomic crises:

Clinton declared a national emergency during the 1996 Cuba embargo, preventing U.S. ships or aircraft from entering Cuban territory without authorization. Obama declared a national emergency during the H1N1 Swine Flu epidemic in 2009 to activate disaster plans to set up proper patient treatment.

Bush declared a national emergency after the terrorist attacks of Sept. 11, 2001; the order is still in effect.

Q: How does a president declare a national emergency?

A: A president must issue a written and signed declaration that specifies the specific emergency powers he plans to rely on and invoke.

"Unlike other executive orders, one that declares a national emergency unlocks the powers contained in more than 100 other laws," Goitein said.

Q: What happens once a national emergency is declared?

A: Even though there aren't many limits on a president's ability to declare an emergency, it does not create carte blanche freedom to act.

Anyone directly affected by the order can challenge it in court. Congress can also draft a concurrent resolution to terminate the state of emergency, leading to a somewhat novel act. Ordinarily, congressional resolutions support a president's declaration of a national emergency.

House Speaker Nancy Pelosi, D-Calif., is expected to bring up a "joint resolution of termination" in the House. Doing so would force Senate Majority Leader Mitch McConnell, R-Ky., to also bring up the resolution in the Senate, putting his members in a difficult position.

Washington Post