NEGOTIATORS FROM the United States, Canada and Mexico are scheduled to meet in Washington on Wednesday in a possible final session to set the stage for House approval of their new trilateral trade agreement . With one of his signature policy initiatives facing a delicate political moment, you might have thought President Trump would refrain from doing anything to rock the boat. No such luck . In pre dawn tweets Monday, Mr. Trump precipitously announced tariffs on steel and aluminum from Brazil and Argentina. On Tuesday, he mused aloud that it might be “better to wait until after the election” in November 2020 to reach a trade truce with China, notwithstanding other messaging from both Beijing and Washington that a deal might be close.

The immediate effect of the president’s outburst was to send stock markets plummeting and to remind the world that the trade policy of the planet’s largest economy is in unsteady hands. Investors can recover their paper losses, perhaps, but it will be hard to restore anything resembling international trust in Mr. Trump’s negotiating after this performance. Nothing is agreed until everything is agreed, goes the old saying; but the president’s action implies that, with him, things may not be agreed even after they are agreed. Just ask Brazilian President Jair Bolsonaro, who has voiced admiration for Mr. Trump and courted him as an ally, only to find his country hit, now, with tariffs based on Mr. Trump’s spurious claim that Brazil has been engaged in a “massive” currency devaluation, “which is not good for our farmers.” Just like that, Mr. Trump blows up a deal he cut with Mr. Bolsonaro’s predecessor to avoid metal tariffs last year, along with a similar arrangement with Argentina. As it happens, Mr. Trump’s accusations of currency devaluation are especially absurd relative to Buenos Aires, which is desperately trying to bolster its currency amid capital flight.

Mr. Trump acted, apparently, under Section 232 of the 1962 Trade Expansion Act, which allows the president to impose tariffs in the name of national security. Notably, his tweet did not even pretend that steel and aluminum imports from Brazil and Argentina actually endanger U.S. security — just “our farmers.” The president has been making promiscuous use of Section 232, but this may be the most egregious example yet. The Cold War-era authors of the measure intended it as an exceptional tool, to be employed in genuine emergencies, not as a routine implement of economic pressure or presidential whim. It is long past time the other branches of government checked Mr. Trump’s abuses of this authority — as the U.S. Court of International Trade attempted to do in a little-noticed but important Nov. 15 ruling allowing a company to sue the government for harm it suffered from an earlier tariff increase on Turkish steel. The president’s power under Section 232 is broad but not unlimited, the court held.

What’s really needed, however, is congressional action, specifically, a rewrite of Section 232 to restore Congress’s constitutional prerogatives over trade. Sen. Charles E. Grassley (R-Iowa), along with other pro-trade senators in both parties, has been drafting a bill to do that. Now would be a good time to redouble that effort.

Read more: