(Jabin Botsford/The Washington Post)

AS IT becomes increasingly apparent that President Trump’s imposition of tariffs on metal products from the United States’ close allies is a strategic policy shift, and not a mere negotiating tactic, the question naturally arises: What, if anything, can be done to prevent it? The allies themselves are planning to push back, both by imposing countervailing tariffs and by presenting a united front against the president’s policies at the Group of 7 summit beginning Friday in Canada. They may also seek redress through legal action at the World Trade Organization.

Yet such efforts may well be costly for all concerned in the short run without succeeding in the long run. Mr. Trump is exploiting a “national security” loophole in international trade law to justify his tariffs, and international tribunals may have little choice but to defer to his sovereign determination in this regard, even if it’s not in good faith. At least one European company and a U.S. counterpart have already sued in a U.S. federal court, claiming that Mr. Trump’s national security rationale is bogus and the tariffs unlawful, but given courts’ usual avoidance of foreign policy issues, that suit is a long shot.

No, if there is going to be effective restraint placed on Mr. Trump’s abuse of his authority in this area, it must come from the same body that delegated it to him in the first place: Congress.

In 1962, Congress passed a landmark statute, the Trade Expansion Act, which gave the president power to negotiate tariff-setting agreements with other countries, as opposed to previous procedures which had involved byzantine tariff-drafting work by lawmakers themselves. As a Cold War-era safeguard, the law permitted a broad “national security” exception, similar to the one in the WTO. No one at the time could imagine a president who would use the power to antagonize allies. Now that we have such a president, the statute should be amended.

Fortunately, there is movement in that direction among Republicans. Sen. Mike Lee (R-Utah), with admirable foresight, proposed a measure on Inauguration Day last year that would require both houses of Congress to sign off on any “unilateral trade action” by the president. His bill has four co-sponsors so far. Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) has announced plans to introduce a similar proposal. Mr. Trump wields a presidential veto, but one way around that might be to attach the measure to the must-pass defense authorization bill.

The Republican default rule heretofore has been to let Mr. Trump have his way, out of fear of antagonizing his voter base, which is also the congressional GOP’s. But if there were ever a time to show some independence and leadership, this is it. The executive branch threatens vital economic and political relationships, requiring a legislative check. Article I, Section 8 clearly assigns the legislative branch the power to “lay and collect . . . duties,” and “to regulate commerce with foreign nations.” It’s time for Capitol Hill to take it back.