US President Donald Trump ruled out attending the Supreme Court hearing on the legality of his tariffs, but his Treasury chief was there / AFP

A majority of US Supreme Court justices appeared deeply skeptical of the legality behind a swath of Donald Trump's tariffs as they heard a landmark case Wednesday that could uphold -- or upend -- the president's economic agenda.

Billions of dollars in customs revenue and a key lever in Trump's trade wars are at stake as the conservative-dominated panel again grappled with the Republican's attempts to expand presidential powers.

The high court's nine justices are considering Trump's use of emergency powers to impose "reciprocal" tariffs on nearly every US trade partner, as well as levies targeting Mexico, Canada and China over their alleged roles in illicit drug flows.

In a hearing lasting more than two-and-a-half hours, several conservative justices, along with the three liberals, questioned whether the International Emergency Economic Powers Act (IEEPA) that Trump invoked confers the authority to impose tariffs.

"The statute doesn't use the word tariffs," said Chief Justice John Roberts, and imposing tariffs is equivalent to taxation, which has always been a "core power of Congress."

The justices sought to clarify whether Congress has to give clear authorization for policies with significant economic or political consequences.

Solicitor General John Sauer, arguing on behalf of the Trump administration, said this did not apply given the president's inherent, broad range of authorities.

"President Trump determined that our exploding trade deficits have brought us to the brink of an economic and national security catastrophe," Sauer said.

Sauer sought to frame the issue as one involving the power to regulate foreign commerce -- including the ability to impose tariffs -- rather than the power to tax.

Justice Sonia Sotomayor, a liberal, noted that the power to impose taxes is a "congressional power, not a presidential power."

"You want to say tariffs are not taxes, but that's exactly what they are," Sotomayor said.

- 'Simply implausible' -

Justice Neil Gorsuch, a Trump appointee, questioned if Congress could reclaim powers once it delegates them to the presidency, suggesting that "as a practical matter in the real world, it can never get that power back."

Neal Katyal, representing small businesses challenging Trump's tariffs, charged that it was "simply implausible" that in enacting IEEPA, Congress "handed the president the power to overhaul the entire tariff system and the American economy in the process."

He contended that the word "regulate" has also not been used to impose taxes.

But there were questions too surrounding refunds if Trump's tariffs were overturned, with Justice Amy Coney Barrett saying it could be "a mess."

The court's decision, which could take months to arrive, does not concern sector-specific tariffs Trump separately imposed, including on steel, aluminum and automobiles.

Trump has brought the average effective tariff rate to its highest since the 1930s, and has repeatedly warned of calamity if his duties are overturned.

A lower court ruled in May that he had exceeded his authority, with the case ultimately making its way to the Supreme Court.

Trump did not attend Wednesday's hearing but several top officials did, including Treasury Secretary Scott Bessent and US Trade Representative Jamieson Greer.

"In recent years, the court has been reluctant to overrule presidential decisions of this magnitude," ING analysts said.

But the outcome is hard to predict, they added, as "upholding Trump's tariffs would shift the balance of power from Congress to the President."

Josh Lipsky of the Atlantic Council noted that even as justices were skeptical, they appeared concerned about fallout from overruling the tariffs.

"How do the refunds work?" Lipsky asked. "What does this mean for the president's foreign policy ability to negotiate deals?"

Although Trump's tariffs have not sparked widespread inflation, companies say they bear the brunt of higher import costs.

Lawyers noted that if the top court finds Trump's global tariffs illegal, the government can tap other laws to temporarily impose up to 15-percent duties while pursuing pathways for more lasting levies.

Countries that have already struck deals with Trump may therefore prefer not to reopen negotiations.

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