The challengers say Trump’s tariffs should get the same treatment, since they’ll have a much greater economic effect, raising some $3 trillion over the next decade. The government, on the other hand, says the tariffs are different because they’re a major part of his approach to foreign affairs, an area where the courts should not be second-guessing the president.

The challengers are also trying to channel the conservative justices’ skepticism about whether the Constitution allows other parts of the government to use powers reserved for Congress, a concept known as the nondelegation doctrine. Trump’s interpretation of the law could mean anyone who can ”regulate” can also impose taxes, they say.

The Justice Department counters that legal principle is for governmental agencies, not for the president.

If he eventually loses at the high court, Trump could impose tariffs under other laws, but those have more limitations on the speed and severity with which he could act. The aftermath of a ruling against him also could be complicated, if the government must issue refunds for the tariffs that had collected $195 billion in revenue as of September.

The Trump administration did win over four appeals court judges who found the 1977 International Emergency Economic Powers Act, or IEEPA, gives the president authority to regulate importation during emergencies without explicit limitations. In recent decades, Congress has ceded some tariff authority to the president, and Trump has made the most of the power vacuum.