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U.S. President Donald Trump reveals a list of tariff rates on ‘Liberation Day’ at the White House on April 2. A series of groups have challenged his ability to impose those levies as well as those on Canada, Mexico and China.Mark Schiefelbein/The Associated Press

Donald Trump’s deployment of tariffs on a wide sweep of U.S. trade is a wide-reaching abuse of presidential authority that violates the country’s Constitution and contravenes legal precedent, lawyers told a federal court Thursday in a renewed attempt to overturn a series of import taxes imposed this year, including fentanyl and migration-related levies on Canada.

The Trump administration has argued that U.S. Congress has delegated immense latitude for the White House to act in response to what it deems an emergency, an argument that a trade court largely rejected in May.

Now, the U.S. Court of Appeals for the Federal Circuit is hearing the case, which turns on balance-of-powers questions so fundamental to the governance of the U.S. that it will almost certainly fall to the Supreme Court to render a final decision. The appeals court, which had earlier issued a stay of the trade court ruling, heard oral arguments this week en banc, with 11 judges present, a recognition of the gravity of the issues.

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What Mr. Trump has claimed is the ability to “do whatever he wants, whenever he wants, for a long as he wants – so long as he declares an emergency,” argued Neal Katyal, a lawyer representing several small importers challenging the tariffs.

“That is as major a question as it gets, a breathtaking claim to power that no president has asserted in 200 years. And the consequences are staggering.”

Congress has passed a series of measures providing circumscribed tariff authority to the President, which Mr. Trump has used to tax imports of copper, steel, cars and auto parts.

But a series of groups have challenged his ability to use the International Emergency Economic Powers Act, or IEEPA, for further tariffs, which include the global “Liberation Day” levies and those he imposed on Canada, Mexico and China on the grounds that those countries were responsible for flows of fentanyl and illegal migration into the U.S.

The text of IEEPA makes no reference to tariffs.

“Tariffs and taxation are always tempting for kings and for presidents, and so that’s why that power was located in Congress. And every single time Congress has departed and given that power to the president, they’ve done so clearly,” Mr. Katyal said.

“If the government is going to have that power, then Congress has to give it to them clearly,” he added. “This is not an elephant in a mouse hole. This is a galaxy in a keyhole.”

Mr. Katyal argued alongside Oregon Solicitor-General Benjamin Gutman, who represents a dozen Democrat-led states that are also challenging the tariffs. Questions from some of the judges suggested support for their arguments.

“IEEPA doesn’t even say tariffs, doesn’t even mention them,” Judge Jimmie Reyna said.

But federal lawyers have argued that a court found then-president Richard Nixon could impose tariffs under a piece of predecessor legislation that shares language with the current act.

The sole limits to that power are that the president must act in an emergency, at a time when there is an extraordinary and unusual threat that emanates from outside the U.S. – and that any action must deal with that emergency.

On Thursday, Brett Shumate, a lawyer acting for the federal government, said Mr. Trump deployed tariffs in response to a genuine series of problems for the U.S.

“In emergencies, Congress wanted to give the President extraordinary authority to protect the United States,” he said.

Mr. Trump, he said, “identified a severe spike in the trading deficit over the last four years. Essentially, it’s reached a tipping point. It’s affecting our military readiness. It’s affecting our domestic manufacturing capability.”

He added: “I know it may be hard to believe that the President may have this power.”

The White House, too, found support for its arguments from some of the judges.

The idea that trade imbalances are damaging core U.S. interests “bothers me,” said Judge Kimberly Moore. “I’m a little concerned about compromised military readiness,” she said.

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The government has argued that courts have little ability to question the President’s judgment in determining what constitutes an emergency.

“A dissatisfaction with the limits in IEEPA itself is not a licence for the court to impose its own limitations on the statute,” Mr. Shumate said.

He argued that Mr. Trump’s use of the tariffs has proven effective, providing the U.S. leverage to negotiate. The White House has now concluded a half-dozen new trade deals, including with major trading partners like the European Union, Britain and Japan.

No deal has yet been reached with Canada or Mexico. On Thursday, Mr. Trump extended a deadline for talks with Mexico by 90 days, hours after warning that Canadian support for Palestinian statehood “will make it very hard for us to make a Trade Deal with them.”

Mr. Trump has continued to deploy tariffs as a tool for negotiation, revenue and protection of U.S. interests in the wake of the initial ruling by the Court of International Trade, which said in May that Mr. Trump acted “contrary to law.”

In a unanimous ruling, three judges, including one appointed by Mr. Trump, ordered that the U.S. lift the tariffs that Mr. Trump placed on Canada and Mexico on fentanyl-related grounds, as well as global tariffs he ordered on April 2, which he called “Liberation Day.” Merely using tariffs as a tool of leverage is not allowed under the law, the trade court wrote.

That order, however, remains stayed pending a decision by the U.S. Court of Appeals for the Federal Circuit.

The White House has argued that judges should have no say in guiding how Mr. Trump addresses what he considers to be urgent economic matters.

“It is not for unelected judges to decide how to properly address a national emergency,” White House spokesman Kush Desai said in May.