The Supreme Court’s six conservative justices have so far been receptive to Mr. Trump’s claims of presidential authority. Among other things, they have allowed the administration to withhold funds appropriated by Congress, kick transgender troops out of the military and pursue aggressive immigration-related policies — but all on a temporary, emergency basis.

The tariffs case is the first time the justices have weighed the underlying legal merits of a key administration priority in Mr. Trump’s second term.