1. What are a president's emergency powers?
There are dozens of laws that give the president special powers to act in an emergency. For example, in an emergency, the president is allowed to divert money to military construction projects, which could conceivably include the border wall.
"Declaring the emergency is pretty easy," said Elizabeth Goitein, co-director of the Liberty and National Security Program at the Brennan Center for Justice. "There aren't a lot of legal limits on his ability to do that, frankly, even if there isn't a real emergency happening."
2. What say does Congress have?
In the mid-1970s, Congress passed a law designed to provide some check on the president's power while also preserving flexibility to act in an emergency. The National Emergencies Act requires the president to notify Congress when an emergency is declared and to list the powers that are being invoked. The law requires the administration to provide regular updates to Congress, and allows lawmakers to terminate an emergency if both House and Senate vote to do so.
3. Would congressional critics have recourse if Trump invoked emergency powers?
Congress could reject the president's emergency declaration with a vote in both the House and Senate. But the Republican-controlled Senate seems unlikely to take that step. Instead, critics would very likely pursue a legal challenge.
"I think the president would be wide open to a court challenge saying, 'Where's the emergency?' " Rep. Adam Smith, D-Wash., chairman of the House Armed Services Committee, told ABC.
Goitein also cautioned that a resort to emergency powers in the current situation — in the midst of a standoff with lawmakers that has produced a partial government shutdown — could be considered an abuse of the president's power.
"Emergency powers are intended to be used for emergencies, not to settle political disputes or to shortcut the political process," Goitein said.
4. What is the emergency?
The president has described the situation on the U.S.-Mexico border as both a humanitarian and national security crisis.
"We have criminals coming in," Trump said Sunday. "We have human traffickers coming in. We have drugs pouring in. We have things happening that you don't want to even know about."
The administration has also raised the specter of terrorists crossing the border, although many of these claims are overblown.
What has happened on the border in recent months is a surge of Central American families seeking asylum. U.S. law and and court orders make it difficult to quickly deport these families or detain them for an extended period while their asylum claims are adjudicated.
5. Would a wall help?
Trump has insisted since the 2016 campaign that a physical barrier along the U.S. border with Mexico is the solution to what he sees as a crisis.
"Walls work," the president wrote in a letter to lawmakers last week. "That's why rich, powerful, and successful people build them around their homes. All Americans deserve the same protection."
Critics say the president's faith in a wall is misplaced.
"It's a spectacularly bad idea, a complete waste of money," Smith told NPR. "The only emergency we have on the border right now is people seeking asylum. And people seeking asylum are not trying to sneak in. They're turning themselves in and asking for a hearing."
NPR's Tamara Keith contributed to this report.
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