At issue is whether the White House exceeded its authority under the International Economic Emergency Powers Act, or IEEPA, when it imposed tariffs of 10% to 50% on dozens of countries.

President Donald Trump has worked fast in his second term, wielding unilateral power to reassert American dominance in the global economy.

If the justices strike them down, "we would be defenseless, leading perhaps even to the ruination of our Nation," Trump wrote in a social media post on Sunday.