Donald Trump’s presidential campaign and his presidency have defined down what is acceptable behavior for a national leader. His willingness to criticize entire ethnic groups, to be blatantly sexist, to mock family members of his opponents and to spread bogus, offensive conspiracy theories may not have slowed his rise to power, but that doesn’t make his behavior acceptable. That’s why then-first lady Michelle Obama’s remarks to the Democratic convention last summer were so welcome. “When someone is cruel or acts like a bully, you don’t stoop to their level,” she said. “No, our motto is: When they go low,...