But the law does not mention tariffs, and no previous president has used it to impose them, according to the Congressional Research Service. The law has been used to impose sanctions on foreign adversaries and wrongdoers — blocking their property and prohibiting transactions with them — for things like terrorism, human rights abuses, malicious hacking, narcotics trafficking, and the proliferation weapons of mass destruction.

What can Congress do about it?

Under the National Emergencies Act of 1976, Congress can legally terminate an “emergency” that a president says exists — and turn off the special powers the president has activated — by enacting a resolution to do so. Senate Republicans apparently threatened to join Democrats in doing just that after a lunch this week in which they were briefed by administration lawyers about the tariffs proposal.

“I will yield to nobody in passion and seriousness and commitment for securing the border,” Senator Ted Cruz, Republican of Texas, told reporters afterward. “But there’s no reason for Texas farmers and ranchers and manufacturers and small businesses to pay the price of massive new taxes” — a burden he estimated would be about $30 billion in his state alone.

But Mr. Trump predicted Republican lawmakers who are openly angry at his plans would back down, saying it would be “foolish” for them to try to block him. And because he can veto such a resolution, at least two-thirds of the lawmakers in each chamber must be willing to vote to override his veto for such a resolution to prevail, meaning many Republicans would have to take the political risk of publicly breaking ranks with Mr. Trump.

What other emergency actions has Trump taken or threatened?

In February, after a standoff with House Democrats over funding for his border wall caused a record-breaking government shutdown, Mr. Trump declared that illegal migration and drug smuggling along the southern border had created a “national emergency” there, enabling him to redirect billions of dollars in Pentagon funds, which Congress had appropriated for other purposes, to border barriers.

That move is currently tied up in court. But Congress separately tried to stop it by passing a resolution to terminate his emergency declaration, with about a dozen Republican lawmakers in each chamber joining Democrats to approve it. Mr. Trump vetoed the resolution, and with most Republicans still sticking with him, an override effort failed to reach the two-thirds threshold to pass the House.

The Trump administration also declared an emergency last month to permit it to circumvent Congress and sell about $7 billion in arms to Saudi Arabia, the United Arab Emirates and Jordan that had been stopped by lawmakers last year, invoking a provision of the Arms Export Control Act of 1976 for national-security exigencies. A bipartisan group of senators announced this week that they would push a set of resolutions to block that move, but their fate was unclear.