Trump Will Declare a National Emergency. What Happens Next?

WASHINGTON — In declaring a national emergency to try to build his long-promised border wall, President Trump will invoke extraordinary and seldom-used authorities to bypass Congress’s constitutionally granted power of the purse.

Democrats are fuming, and many members of Mr. Trump’s party are uneasy about the precedent that such a declaration will set. Lawmakers seeking to block the president have two paths — one in Congress, the other in the courts.

Democrats, who control the House, will drive the opposition. Party leaders did not detail their plans on Thursday, as they reviewed options and awaited the text of the president’s declaration. But outspoken senior lawmakers indicated that they will likely use both paths at once.

“He may want to talk to a good lawyer,” said Senator Patrick J. Leahy of Vermont, the top Democrat on the Appropriations Committee.

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 resolution of termination to end the emergency status if they believe the president is acting irresponsibly or the threat has dissipated. The House has signaled that it intends to do that in the coming weeks, and with a comfortable majority in the chamber, Democrats will most likely pass it.

“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.

As with any other bill that comes to the president’s desk, Mr. Trump can veto a joint congressional resolution terminating the national emergency, as long as it has not passed with supermajorities in both the House and the Senate.

Congress did not originally intend to give the president this recourse when it enacted the law during the post-Watergate reform era that has governed how and when presidents may invoke emergency-power statutes.

But the Supreme Court struck down what it calls legislative vetoes in 1983, ruling that for a congressional act to take legal effect, it must be presented to the president for signature or veto. Because it takes two-thirds of both chambers to override a veto, the ruling made it substantially harder for Congress to stop a president’s declaration.

This is the crucial question. Half a dozen or so Republicans made their disapproval of an emergency declaration clear on Thursday.

“I don’t believe that the National Emergencies Act contemplated a president repurposing billions of dollars outside the normal appropriations process,” said Senator Susan Collins, Republican of Maine. “I also believe it will be challenged in court and is of dubious constitutionality.”

Senator Rand Paul, Republican of Kentucky and an ally to the president on other issues, said a national emergency declaration of this sort ran counter to the separation of powers outlined in the Constitution. Senator Patrick J. Toomey, Republican of Pennsylvania, agreed.

“My view is that this is better to be resolved through the legislative process,” he said.

Still, it is highly unlikely that Democrats can pick up enough Republican supporters in the House or the Senate to override Mr. Trump’s likely veto. The best case for the president’s opponents is that they show deep fissures among Republicans over the wall.

They could also try to build bipartisan support for legislation preventing Mr. Trump from drawing money for the wall from funds allocated by Congress for disaster relief.

Here Democrats appear to have two choices. The House could either support a lawsuit challenging the emergency declaration brought by a third party or file a suit of its own.

There are questions about the House’s legal standing if it were to try to sue on its own. Regardless, a legal challenge is likely to tie up Mr. Trump’s efforts in court for an extended period of time.

Charlie Savage and Catie Edmondson contributed reporting.