President Donald Trump’s endless provocations have driven his critics to a state of constant outrage, and for some time the Resistance has found it difficult to distinguish between what does and doesn’t matter. What Trump did last week matters. His decision to fire the first shot in what could become a trade war against U.S. allies demands a whole new level of alarm about the damage this president might do.

This is a major new policy, not some theatrical maneuver, empty threat, vulgar insult or norm-shaking tweet. And this policy departure, over which Trump has improperly asserted unilateral political control, runs counter to decades of U.S. thinking on international economic relations. Congress has the responsibility and the power to keep it from happening.

Decades from now, historians might be amazed that Trump chose to disdain U.S. alliances just as China’s ambitions as an ideological rival and emerging military superpower had come more sharply into focus. They might be puzzled, too, that Washington at the time was preoccupied with questions such as whether it’s correct to call a covert informant a spy.

Of course, Trump might not persist. He’s nothing if not erratic. But then, few had expected him to carry out his earlier threat to put tariffs on imports of steel and aluminum from Canada, Mexico and Europe, and he has. All three have said they’ll retaliate with tariffs, doubtless chosen to maximize Trump’s embarrassment. U.S. businesses, facing higher costs, are already complaining. But what if Trump then doesn’t back off? A cycle of retaliation and counter-retaliation looks possible. 

The harm to a highly integrated global economic system could be immense, especially if businesses see these measures as the start of a protracted unraveling of the liberal economic order (which in fact appears to be Trump’s goal). And with broader alliances at stake, the economic harm is only part of the cost. As European Council President Donald Tusk asked last month, “With friends like that, who needs enemies?”

You’d surely expect to see more local resistance—actual resistance, I mean, as opposed to the Resistance that Trump finds so empowering. Congress should be pressing to take back control of trade policy. This could and should be a bipartisan endeavor.

Hard to believe, but the Constitution vests Congress with the power to regulate international commerce. The White House has a narrow role delegated to it by statute, but it has no general authority to modify tariffs. Trade Promotion Authority, for instance, was granted to let the president negotiate international agreements on a fast track—the idea being, you know, to promote trade. Trump has the opposite intention. And the tariffs he announced last week are under the so-called Section 232 delegation intended to allow for unimpeded action in cases where national security is threatened.

The idea that steel imports from Canada raise national-security concerns is fatuous. U.S. domestic capacity exceeds defense needs by a factor of 30. But the president has even talked of invoking the same rationale for new tariffs on cars. In other words, he and his commerce secretary believe that national security provides a rationale for any and every random protectionist measure.

Imports of cars threaten national security only in the expansive sense that they could be said to undermine the economy—which they don’t, obviously. Unfortunately, the law does leave room for this very broad interpretation: It tells the president to determine whether a “weakening of our internal economy may impair the national security.” But if presidents take this as a license to raise tariff barriers and restrict trade as they see fit—a policy that will severely impair the national economy—then Congress has as good as removed itself from trade altogether.

The Republican Party is traditionally pro-trade. It’s safe to assume that most Republicans in Congress think the president’s approach is misconceived. But only a handful of GOP senators appear ready to support legislation to rein in Trump’s freedom of action on tariffs. Such action is apparently deemed unwise ahead of the mid-term elections. Democrats aren’t pressing for this remedy as hard as one might wish, either—indeed many of them have said the president is right to be cracking down on imports.

In effect, Congress is leaving it to Trump to decide whether U.S. trade policy will wound the U.S. economic expansion slightly, or flatten it completely. It’s an abdication of Congress’s responsibility under the Constitution—and could be the most consequential dereliction of duty so far.