What could happen next?

 The White House says that if it loses, it will impose levies via other means, such as a law allowing the president to put tariffs of up to 15% in place for 150 days.

Even then, businesses would have some relief, since those other means require steps like issuing formal notices, which take time and deliberation, said trade lawyer Ted Murphy of Sidley Austin.

"This is not just about the money," he said. "The president has announced tariffs on Sunday that go into effect on Wednesday, without advance notice, without any real process."

"I think that's the bigger thing for this case for businesses - whether or not that is going to be in our future," he added.

There is no clear sign of how the court will rule.

In recent years it has struck down major policies, such as Biden-era student loan forgiveness, as White House overreach.

But the nine justices, six of whom were appointed by Republicans, including three by Trump, have shown deference to this president in other recent disputes and historically have given leeway to the White House on questions of national security.

"I really do think arguments are available for the Supreme Court to go in all different directions," said Greta Peisch, partner at Wiley and former trade lawyer in the Biden administration.

Adam White, senior fellow at the American Enterprise Institute, said he expected the court to strike down the tariffs, but avoid questions like what constitutes a national emergency.

Reuters

European Commission President Ursula von der Leyen and Trump announcing a deal in July

The case has already complicated the White House's trade deals, such as one struck in July with the European Union.

The European Parliament is currently considering ratifying the agreement, which sets US tariffs on European goods at 15% in exchange for promises including allowing in more US agricultural products.

"They're not going to act on this until they see the outcome of the Supreme Court decision," said John Clarke, former director for international trade at the European Commission.

Chocolats Camille Bloch

Swiss chocolatier Daniel Bloch says he is not confident the Supreme Court will resolve the tariff issues facing his business

In Switzerland, which recently downgraded its outlook for economic growth citing America’s 39% tariff on its goods, chocolatier Daniel Bloch said he'd welcome a ruling against the Trump administration.

His business Chocolats Camille Bloch is absorbing about a third of the cost of new tariffs on kosher chocolate that his firm has exported to the US for decades, aiming to blunt price increases and maintain sales. That decision has wiped out profits for the unit and is not sustainable, he said.

He hopes Trump will reconsider his tariffs altogether, because "that would be easiest".

"If the court were to make the tariffs go away of course we would see that as a positive sign," he said. "But we don't trust that that will bring the solution."