Vice President Pence, right, listens as President Trump speaks at the White House on March 8. (Susan Walsh/AP)

PRESIDENTIAL POWER over trade and tariff policy is a little-known but crucial, and enduring, legacy of the New Deal. Reacting to the disastrous Smoot-Hawley Tariff enacted by a Republican Congress in 1930, the Democratic Congress under President Franklin D. Roosevelt passed legislation empowering the executive branch to set tariffs through reciprocal bargains with other nations. For more than eight decades, this broad delegation of Congress’s constitutional authority to “lay and collect” tariffs has basically remained in place. And, with certain emergency exceptions, presidents have by and large used it to reduce tariffs, through both bilateral and multilateral agreements that were subject to majority votes in both houses of Congress, rather than the two-thirds of the Senate required for treaties.

Among many reasons President Trump’s declaration of new tariffs on steel and aluminum, ostensibly on national security grounds, represents a potentially radical departure in U.S. trade policy is that it shows what can happen when this legal authority, designed for an era of trade-promoting presidents, suddenly falls into the hands of a protectionist. Congress needs to respond accordingly, before the conflict with trading partners, still manageable, produces irreversible negative consequences.

Sen. Jeff Flake (R-Ariz.) has stepped forward with a simple 2½-page bill prohibiting the enforcement of Mr. Trump’s tariffs. It’s clearly within Congress’s purview, constitutionally; politically, though, the bill is a non-starter, given that Mr. Trump would veto it in the unlikely event it passed. On the very first day of the Trump administration, Sen. Mike Lee (R-Utah) — sensing what was coming — introduced the Global Trade Accountability Act, which would require the president to get approval from both houses of Congress before any “unilateral trade action” could take effect, subject to temporary exceptions for “national emergencies.”

If Mr. Lee’s bill had been the law, Mr. Trump would now be lobbying Capitol Hill for support of his tariffs, rather than entertaining lobbyists from all of the countries — and U.S. companies — that will be disadvantaged by them. The Republican leadership of Congress is taking a wait-and-see approach to the president’s reckless policies, hoping that the exemptions he has offered Canada, Mexico and, more conditionally, other allies prove to have substance and permanence.

Perhaps this is prudent, given that the president’s thinking is malleable and that restoring all tariff-setting power to Congress has drawbacks of its own. In pre-New Deal times, lobbyists swarmed the halls of Congress as members drafted minutely detailed tariff schedules. Smoot-Hawley covered 173 pages after 18 months of drafting. A balance must be struck between curtailing Mr. Trump’s excesses and tying the hands of future presidents.

At some point, however, Republican defenders of free trade will have to decide between their declared principles and this president. It’s a faint hope, but, for the future of international trade, possibly the best one.