Mr. Trump has said that, if elected, he would instruct his attorney general to reopen criminal investigations into Mrs. Clinton’s use of a private email server. (After the election, he appeared to soften his position, saying, “I don’t want to hurt the Clintons.”) Mr. Trump will have the legal power to tell his attorney general to open or close investigations, though doing so would violate political norms. But his suggestion that the president has the unilateral power to jail people for federal crimes overstates the president’s authority. Presidents acting alone do not adjudicate criminal cases or send people to jail. By contrast, the Constitution does grant the president the unilateral power to show leniency.