WASHINGTON (Reuters) -U.S. Supreme Court justices raised doubts on Wednesday over the legality of President Donald Trump's sweeping tariffs in a case with implications for the global economy that marks a major test of Trump's powers.

Conservative and liberal justices alike sharply questioned the lawyer representing Trump's administration about whether a 1977 law meant for use during national emergencies gave Trump the power he claimed to impose tariffs or whether the Republican president had intruded on the powers of Congress.

But some of the conservative justices also stressed the inherent authority of presidents in dealing with foreign countries, suggesting the court could be sharply divided in the outcome of the case. The court has a 6-3 conservative majority.

The arguments, lasting more than 2-1/2 hours, came in appeals by the administration after lower courts ruled that Trump's unprecedented use of the law at issue to impose the tariffs exceeded his authority. Businesses affected by the tariffs and 12 U.S. states, most of them Democratic-led, challenged the tariffs. 

'CORE POWER'

Conservative Chief Justice John Roberts told U.S. Solicitor General D. John Sauer, arguing for the administration, that the tariffs are "the imposition of taxes on Americans, and that has always been the core power of Congress."

The tariffs - taxes on imported goods - could add up to trillions of dollars for the United States over the next decade. The U.S. Constitution gives Congress the authority to issue taxes and tariffs. 

Roberts suggested that the court could apply its "major questions" doctrine, which requires executive branch actions of vast economic and political significance to be clearly authorized by Congress. 

"The justification is being used for a power to impose tariffs on any product, from any country, in any amount, for any length of time. I'm not suggesting it's not there, but it does seem like that's major authority, and the basis for that claim seems to be a misfit," Roberts said.

The Supreme Court applied the "major questions" doctrine to strike down key policies of Trump's Democratic predecessor Joe Biden.

Trump has heaped pressure on the Supreme Court to preserve tariffs that he has leveraged as a key economic and foreign policy tool. A ruling against Trump would mark a significant departure for the court, which has backed him in a series of decisions allowing on an interim basis his far-reaching actions in areas as varied as his crackdown on immigration, the firing of federal agency officials and banning transgender troops.

Trump invoked the International Emergency Economic Powers Act, or IEEPA, to impose the tariffs on nearly every U.S. trading partner. The law lets a president regulate commerce in a national emergency. He became the first president to use IEEPA for this purpose, one of the many ways he has pushed the boundaries of executive authority since returning to office in January.

Conservative Justice Amy Coney Barrett questioned Sauer about his claim that IEEPA's language granting presidents emergency power to regulate importation encompasses tariffs.

"Can you point to any other place in the code or any other time in history where that phrase together 'regulate importation' has been used to confer tariff-imposing authority?" Barrett asked Sauer.

Liberal Justice Ketanji Brown Jackson said IEEPA was intended to limit presidential authority, not expand it. 

"It's pretty clear that Congress was trying to constrain the emergency powers of the president," Jackson said.

While the Supreme Court typically takes months to issue rulings after hearing arguments, the administration has asked it to act swiftly in this case, though the timing of the decision remains unclear.

U.S. Treasury Secretary Scott Bessent, who attended the arguments on Wednesday, has said that if the Supreme Court rules against Trump, the administration would switch to other legal authorities to ensure his tariffs remain in place. Trump has imposed some additional tariffs invoking other laws. Those are not at issue in this case.

'RUTHLESS TRADE RETALIATION'

IEEPA gives the president power to deal with "an unusual and extraordinary threat" amid a national emergency. It historically had been used for imposing sanctions on enemies or freezing their assets.

Sauer said Trump determined that U.S. trade deficits have brought the nation to the brink of an economic and national security catastrophe, and that tariffs have helped Trump negotiate trade deals. Unwinding those deals "would expose us to ruthless trade retaliation by far more aggressive countries and drive America from strength to failure with ruinous economic and national security consequences," Sauer said. 

Trump instigated a global trade war when he returned to the presidency, alienating trading partners, increasing volatility in financial markets and fueling global economic uncertainty. 

He has wielded tariffs to extract concessions and renegotiate trade deals, and as a cudgel to punish countries on non-trade political matters. 

Trump invoked IEEPA in slapping tariffs on goods imported from individual countries to address what he called a national emergency related to U.S. trade deficits, as well as in February as economic leverage on China, Canada and Mexico to curb the trafficking of the often-abused painkiller fentanyl and illicit drugs into the United States.

MAJOR QUESTIONS DOCTRINE

Emphasizing the president's powers in the realm of foreign affairs, Sauer told the justices that the "major questions" doctrine should not apply in this context.

Conservative Justice Brett Kavanaugh signaled potential sympathy for Trump, pushing back on the argument that Trump had done something novel that could implicate the doctrine. Kavanaugh noted that President Richard Nixon imposed a worldwide tariff under IEEPA's predecessor statute in the 1970s that contained similar language regarding regulation of importation. 

"That's a good example for you," Kavanaugh told Sauer. 

The Supreme Court has long shown deference to the president on conducting foreign policy. Roberts, while questioning a lawyer for the private business challengers, Neal Katyal, seized upon this point, noting that Trump's tariffs have undoubtedly given him leverage in making foreign trade agreements.  

The IEEPA-based tariffs generated $89 billion in estimated collections between February 4 and September 23, when the most recent data was released by the U.S. Customs and Border Protection agency. 

"Sure, the tariffs are a tax, and that's a core power of Congress, but they are a foreign-facing tax, right? And foreign affairs is a core power of the executive," Roberts told Katyal. 

Questions posed by conservative Justice Neil Gorsuch suggested that he thinks Sauer's claims about the breadth of the president's inherent foreign affairs powers would threaten to undermine the Constitution's separation of powers between the federal government's executive and legislative branches.

"What would prohibit Congress from just abdicating all responsibility to regulate foreign commerce - or for that matter, declare war - to the president?" Gorsuch asked.