The justices are set to weigh a pair of legal challenges to Trump’s use of emergency powers to impose tariffs on several countries by invoking a nearly 50-year-old law. No president before Trump has used the law, known as the International Emergency Economic Powers Act, to impose tariffs, which have brought in tens of billions of dollars to the U.S. government.

Trump’s decision came after at least one prominent Trump ally indicated it would be unwise for the president to attend.