STEVE INSKEEP, HOST:
One thing the president did not mention last night was a national emergency. The White House has said he is thinking of declaring one. He could then divert money from other programs to wall construction. To do that, the president would invoke the National Emergencies Act, which was passed by Congress in the 1970s.
That same law is supposed to give Congress a way to take back emergency powers when a president claims them. But Congress never has. NPR's David Welna reports.
DAVID WELNA, BYLINE: It might seem counter-intuitive that Congress would give a president free rein to use emergency powers. Not really, says Elizabeth Goitein.
ELIZABETH GOITEIN: The idea behind emergency powers is a pretty simple one.
WELNA: Goitein, who co-directs the Brennan Center's Liberty and National Security Program, says Congress recognizes that sometimes speed is of the essence, while enacting new laws can be anything but speedy.
GOITEIN: The concern is that in an emergency - in a true emergency, which is unforeseen and unforeseeable - the laws that are on the books might not be sufficient to deal with the emergency. It's this idea that extraordinary times call for extraordinary measures.
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UNIDENTIFIED REPORTER: And you can see the two towers - a huge explosion now raining debris on all of us. We'd better get out of the way.
WELNA: When hijacked airliners crashed into the World Trade Center, Goitein says, then-President George W. Bush declared a national emergency, which is still in effect.
GOITEIN: That was partly because no one really knew what was coming next. It was unfolding very quickly. There was a sense that the president might have to move extremely quickly, without getting Congress' authorization.
WELNA: Bush suddenly had powers that Goitein says would otherwise be illegal. All that is thanks to the National Emergencies Act. Stephen Vladeck is an expert on national security law at the University of Texas.
STEPHEN VLADECK: When the National Emergencies Act was enacted in 1976, the way that Congress set it up was that Congress could basically terminate any national emergency the president declared through a concurrent resolution, simply through majority votes of both houses, without the president's approval.
WELNA: But a few years later, the Supreme Court said the president should be able to veto such a resolution, which would require a two-thirds majority in both chambers to override. But so far, that's moot. Again, Elizabeth Goitein.
GOITEIN: Congress has never voted once, in the last 40 years since the National Emergencies Act has been in effect, to terminate a state of emergency.
WELNA: In part, says law professor Vladeck, that's because Congress has trusted that presidents won't abuse those powers.
VLADECK: The assumption behind all of these regimes is that presidents are going to be relatively responsible in using those authorities and resources and are not going to just create some kind of pretext to allow them to go through a back door when Congress is denying them the front door.
WELNA: There are 31 national emergency decrees currently in effect. Nearly all have been used to freeze foreign assets. The longest has been standing for 40 years - a freeze on Iranian assets imposed by President Jimmy Carter. NYU's Goitein says Trump would break new ground for an American president if he were to invoke a national emergency to build a wall.
GOITEIN: It would be an enormous abuse of his powers under the National Emergencies Act. Emergency powers are not supposed to be used just to implement policy preferences.
WELNA: If Congress does nothing to block a national emergency decree, the University of Texas' Vladeck says it's not clear whether the courts would either.
VLADECK: Congress didn't define what is and what is not a national emergency. And so it's hard to imagine what criteria a federal court could use in trying to decide whether a national emergency was properly declared or not.
WELNA: Should Trump invoke the National Emergencies Act, he'd have to specify which statutes he'd use to build a border wall. Todd Harrison is a military budget expert at the Center for Strategic and International Studies. He says Trump is likely to tap powers to re-program available funds for military construction. That, he adds, could be a problem.
TODD HARRISON: Border security is the responsibility of the Department of Homeland Security, not the Department of Defense. So I think it is not clear at all that the declaration of a national emergency here would actually allow the administration to use military funding for a non-military purpose.
WELNA: Ultimately, Harrison says, it may be the landowners and local officials affected by the wall's construction who would sue to stop it. David Welna, NPR News.