Portside

A powerful sign that President Trump’s tariff-driven trade war is at risk came in a friend-of-the-court brief filed in April by a coalition that included many prominent conservative and libertarian lawyers, scholars and former officials.

The brief was also a signal of a deepening rift between Mr. Trump and the conservative legal movement, one that burst into public view last week with the president’s attacks on the Federalist Society, whose leaders helped pick the judges and justices he nominated in his first term.

Among the people who signed the brief in the tariffs case was Richard Epstein, who teaches at New York University and is an influential libertarian legal scholar.

“You have to understand that the conservative movement is now, as an intellectual movement, consistently anti-Trump on most issues,” he said.

Others who signed the brief, filed in the U.S. Court of International Trade, included Steven G. Calabresi, a founder of the Federalist Society; Michael B. Mukasey, a former federal judge who served as attorney general under President George W. Bush; and three former Republican senators — George F. Allen, John C. Danforth and Chuck Hagel. The brief was signed by liberals, too, including Harold Koh, a former dean of Yale Law School.

“The brief unites big-name constitutional law scholars across the political spectrum in a way I have rarely seen,” said Ilya Somin, a law professor at George Mason University and a lawyer for a wine importer and other businesses that sued over the tariffs.

“I never would have expected to see Richard Epstein, Steve Calabresi and Harold Koh all on the same brief on a major issue,” he said. “But here they are, together, opposing ‘taxation by proclamation.’ Donald Trump brought them together.”

The brief was prepared by Michael W. McConnell, a former federal appeals court judge appointed by Mr. Bush who teaches at Stanford Law School, and Joshua A. Claybourn, a lawyer and historian. It said Mr. Trump’s program did violence to the constitutional structure.

“The powers to tax, to regulate commerce and to shape the nation’s economic course must remain with Congress,” the brief said. “They cannot drift silently into the hands of the president through inertia, inattention or creative readings of statutes never meant to grant such authority. That conviction is not partisan. It is constitutional. And it strikes at the heart of this case.”

The coalition filed a very similar brief in a second case, too, in the Federal District Court in Washington. On Wednesday, the trade court ruled for the challengers. On Thursday, the district court judge followed suit, citing the supporting brief eight times.

Professor McConnell said that the prominence of the amici curiae — the friends of the court — who signed the brief sent a message.

“Our hope is that the identity of the amicus parties will signal the gravity of the situation,” he said. “All are concerned about executive usurpation of control over taxation of trade, which the Constitution explicitly vests in Congress.”

Professor Koh, who served as a State Department official in the Obama administration, said some issues transcend partisanship.

“Despite our political differences, the amici easily agreed, as lawyers, that the president has exceeded his delegated statutory authorities,” he said. “By unilaterally imposing unlimited tariffs on worldwide goods, he has lawlessly usurped Congress’s exclusive powers to impose taxes and duties and to regulate foreign commerce.”

The brief said it took no position on the wisdom of the tariffs.

“Amici do not appear to defend or oppose any particular trade policy,” it said. “They file this brief because they believe the Constitution draws bright lines between legislative and executive power — and that those lines are being blurred in ways that threaten democratic accountability itself.”

Professor Epstein said he had been honored to sign what he called a magnificent document, one that boiled the dispute down to its essence.

“This case is not close,” he said. “There are cases that are vastly important that are easy.”

Other scholars have offered more cautious assessments. Jack Goldsmith, a law professor at Harvard who was a Justice Department official in the Bush administration, recently wrote that the legal issues in the case were “hard and close.”

Karoline Leavitt, the White House press secretary, went further. “The president’s rationale for imposing these powerful tariffs was legally sound and grounded in common sense,” she said at a briefing last week.

An appeals court has temporarily paused the trade court’s ruling and will consider whether to extend that pause in the coming days. There is little doubt that the case will reach the Supreme Court, and soon.

When it does, the justices will have to grapple with two doctrines dear to the conservative legal movement, both of which would seem to cut against Mr. Trump’s understanding of his powers.

One, the nondelegation doctrine, says that Congress may not transfer unbounded legislative powers to the executive branch. The other, the major questions doctrine, says Congress must authorize in plain and direct language any sweeping executive actions that could transform the economy.

The friend-of-the-court brief said those doctrines, grounded in the separation of powers, required courts to reject Mr. Trump’s program.

“This case presents the court with a choice — not between competing trade policies, but between rival understandings of constitutional governance,” the brief said. “One preserves the balance the framers struck, requiring that major economic decisions receive explicit legislative authorization. The other would allow the executive to unilaterally remake the nation’s commercial framework under vague and general statutory language never intended to support such action.”

“The court,” the brief said, “should choose the former.”

Adam Liptak covers the Supreme Court and writes Sidebar, a column on legal developments. A graduate of Yale Law School, he practiced law for 14 years before joining The Times in 2002.

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