Last week’s arrest of an African-American liberal for phoning in antisemitic bomb threats to American Jewish Community Centers seemingly ruined two popular narratives. It proved that American antisemitism is not a Donald Trump-driven right-wing phenomenon.It also implied that having President Trump denounce antisemitism wasn’t as important as many suggested.The first corrective is correct: antisemitism is the world’s all-purpose prejudice. Just as European antisemites hated Jews for being Rothschild-like capitalists and Marx-like communists, American antisemitism has become an altright and alt-left disorder. But the second corrective is incorrect: whether Trump would have swayed this particular criminal is irrelevant. Leadership counts, words matter, gestures speak loudly, in fighting Jew hatred or any prejudice, and America’s president can help win the battle.Trump’s moral leadership in repudiating bigotry – albeit belatedly – is important because bigotry is a conceptual crime and a social disease. The Rogers and Hammerstein song from South Pacific is correct: “You’ve got to be taught before it’s too late/ Before you are six or seven or eight/ To hate all the people your relatives hate.” No one is born hating people who practice another religion or have another skin color – a minor genetic variation some humans exaggerate into major moral differences. As America’s national teacher, the president helps shape attitudes on many things, especially something as fluid as prejudice.Furthermore, bigotry is a social disease affected by peer pressure and groupthink. Consider all the animal pack metaphors used when discussing prejudice. When we say bigots are “dog whistling” subtly or “hounding” overtly, because it’s “open season” on a particular group; when we say leaders should “call off the dogs” or “hold haters at bay,” we acknowledge the power of the group and the group leader. As America’s national preacher, the president can legitimize some behaviors and delegitimize others.Fortunately, America’s presidents have an impressive track record in fighting antisemitism. In 1790, president George Washington reassured the Hebrew Congregations of Newport, Rhode Island, that “the Government of the United States gives to bigotry no sanction, to persecution no assistance.” With Jews more accustomed to leaders assisting persecutors, not assailing them, Washington’s embrace was extraordinary.A century later, before becoming president, Theodore Roosevelt was even more aggressive – and clever – in fighting Jew hatred, demonstrating what Vice President Mike Pence’s visit to the St. Louis Jewish cemetery to fix destroyed tombstones showed – that gestures can speak very eloquently. When Roosevelt was New York’s police commissioner, an antisemitic German preacher visited New York.Resisting requests to ban the speech to avoid making Rector Hermann Ahlwardt a free speech “martyr,” TR ordered Ahlwardt protected by the very people he detested – dozens of Jewish policemen. In his autobiography, TR explained that he made this bigot “ridiculous” to teach an “object-lesson”: Americans must “judge each individual on his own conduct and merits.”This wealthy New York Republican and passionate nationalist demonstrated that Americanism should be welcoming – and that what he would call as president the White House “bully pulpit” shouldn’t become a bullying pulpit. Fortunately, most of TR’s successors added their own links to this chain of presidential decency. William Howard Taft was the first president to attend a seder – decades before there were White House Hanukka parties and seders. Woodrow Wilson appointed the first Jew to the Supreme Court – despite being warned that antisemites would fight Louis Brandeis’s nomination. And in April 1945, Dwight Eisenhower – while still a general – forced all the German residents of the town of Gotha to visit the neighboring Ohrdruf concentration camp to rebuke them for enabling the “overpowering.... starvation, cruelty, and bestiality,” he witnessed at the camp his troops had just liberated.It’s silly to accuse Trump of being antisemitic for dithering before denouncing antisemitism. His hesitation reflected a more fundamental flaw: except for his speech before Congress, he has failed to understand that, as Franklin Roosevelt explained, “the presidency is preeminently a place of moral leadership.” From his prickly inauguration speech to his nightly Twitterhea, Trump has preferred the leadership low road to the high road.Trump’s in-your-face antics command media coverage – and national attention – as if he had put everyone under the hypnotic, brainwashing “Imperius Curse” from the Harry Potter series. And Trump’s smallness as a leader so far reflects the smallness of today’s self-indulgent, celebrity culture. His campaign soared thanks to America’s cynical postmodernism that scorns ideals.Yet the positive polls following his speech showed how anxious Americans are for ennobling leadership – and how quick Americans will be to forgive other transgressions, if Trump acts statesmanlike.Meanwhile, all those lecturing Trump should follow their own advice. University presidents and professors should condemn campus Jew hatred with the passion they demanded from Trump – even when the Jew haters are from the Left, and even if they mask their antisemitism with human rights rhetoric about Israel’s treatment of the Palestinians. Zero tolerance on campus for all bigotry except Jew hatred encourages Jew hatred.And rather than just looking up to those in authority to hate haters, Americans should remember the democratic teaching that individuals are ultimately responsible for their society, not their leaders. Every one of us, in America and beyond, must stop relying on Big Brother – or Sister. Let’s take responsibility to fight bigotry passionately, consistently and effectively, especially from our political allies, where our objections will have more credibility – and impact. The author, a Distinguished Scholar in North American History at McGill University and a Visiting Professor at the Ruderman Program at Haifa University, is the author of The Age of Clinton: America in the 1990s, published by St. Martin’s Press. His next book will update Arthur Hertzberg’s The Zionist Idea. Follow on Twitter @GilTroy.