The 6-3 ruling by the justices was a stinging defeat for Mr. Trump and a relatively rare instance of the court imposing a check on efforts in his second term to expand presidential power. Mr. Trump had repeatedly cast the tariffs as essential to his agenda, and his loss at the court will affect not only trade but his entire economic and foreign policy approach — he has repeatedly used tariffs, and the threat of them, to bludgeon both adversaries and allies.