On the night he won a second term, President-elect Donald J. Trump rejoiced in the moment. “America has given us an unprecedented and powerful mandate,” he boasted. In the two weeks since, his campaign has repeatedly heralded his “landslide,” even to market Trump merchandise like the “Official Trump Victory Glass.”But by traditional numeric measures, Mr. Trump’s victory was neither unprecedented nor a landslide. In fact, he prevailed with one of the smallest margins of victory in the popular vote since the 19th century and generated little of the coattails of a true landslide.The disconnect goes beyond predictable Trumpian braggadocio. The...