Can Congress stop Trump from declaring an emergency?

No, Congress does not have the power to stop the president from declaring a national emergency. But when lawmakers granted the president emergency powers in the first place, they built a check into the law.

Under the National Emergencies Act, the House and the Senate can take up what is called a joint resolution of termination to end the emergency status if they believe the president is acting irresponsibly or the threat has dissipated. Representative Joaquin Castro, Democrat of Texas and the head of the Congressional Hispanic Caucus, said late Thursday that he was ready to introduce such a resolution if Mr. Trump followed through. With a comfortable majority in the chamber, Democrats will most likely pass it or a similarly worded resolution.

“I will fully support the enactment of a joint resolution to terminate the president’s emergency declaration, in accordance with the process described in the National Emergencies Act, and intend to pursue all other available legal options,” said Representative Jerrold Nadler of New York, the chairman of the Judiciary Committee.

To keep a president’s party from bottling such a measure up, the law says that if one chamber passes such a resolution, the other one must bring it up for a vote within 18 days. Though Democrats are in the minority in the Senate, they would need only a handful of Republicans to join them to pass the resolution there and send it to Mr. Trump’s desk. It is easy to imagine a half-dozen or more Republican senators joining Democrats out of concern for the precedent that Mr. Trump’s declaration will set.

What would Mr. Trump do next?

As with any other bill that comes to the president’s desk, Mr. Trump can veto a joint congressional resolution terminating the national emergency, though supermajorities in both the House and the Senate could ultimately override him.