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.