It sounds like an old antisemitic joke. Alas, it’s becoming American Jews’ new reality. Blacks and whites clash over a Confederate statue in Virginia. The neo-Nazis and KKK marchers occasionally replace their antiblack, anti-immigrant, White Power slogan “you will not replace us” with the cry “Jews will not replace us.” Meanwhile, in Chicago that same day, a feminist “Slutwalk” protesting sexual violence features a Palestinian speaker delivering the non-sequitur “You cannot be a Zionist and a feminist.”It is of course an old story – the laziness of the bigot. Once upon a time, when you were not sure who to kick, you kicked the Jew. Today, when you are not sure what to bash – you bash Israel.For centuries, European antisemites spiked whatever cause they led with Jew-hatred, making antisemitism the demagogue’s all-purpose brew. While antisemitism arrived in the New World in 1654 along with the first Jews, American antisemitism traditionally was milder, more personal, less politically potent, less public, than the European variety. Today’s politicizing of American antisemitism reflects Americans’ growing addiction to a politics of insult instead of inspiration. Building the first liberal nation, many Americans have taken pride in welcoming all, especially the long-persecuted Jewish People. More cynically, blacks have been America’s Jews: the scapegoats, the lightning rods, the intimate yet detested “others.” For decades, Southern politicians competed to see who yelled the N-word the loudest. By contrast, Jew-baiting rarely defined an American election.By the 1960s and 1970s, with Jews now considered white – no longer “of the Jewish race” – right-wing authoritarian antisemitism was flagging. The moral stain of Auschwitz also inhibited Jew haters. In 1991, the grand old man of the American Right, William F. Buckley, wrote In Search of Anti-Semitism, a stinging rebuke of Jew hatred. Buckley would have been particularly dismayed to see the alt-right’s sweeping attacks on Jews as ultra-liberals boosting their non-white enemies.Ironically, as the Republican Party repudiated antisemitism, New Left anti-Zionism emerged. Many of these progressive bigots insist they only hate Israel, not Jews. It’s like saying “I only hate feminists, not women”: theoretically possible, but rarely achieved. In practice, where anti-Zionism festers, antisemitism inevitably flows. The obsessive focus on Israel, exaggerating it into the chief international villain, is too reminiscent of the medieval obsession with the Jew. And Israel boycott proponents all too easily use classic antisemitic terms like “Jew boy,” as happened recently at McGill – or bully Jews, not “just” Zionists.Here, then, is the mystery: in an America so polarized many fear they no longer share common ground, how can both extremes agree on antisemitism? As what the historian Robert Wistrich called “The Longest Hatred,” antisemitism offers a wide array of slurs for haters to deploy. Jews have been detested for being too powerful and too pitiful, too rich and too poor, too capitalist and too Marxist, too religious and too secular, too eager to assimilate and too unassimilable.For such a protean hatred, being politically ambidextrous in the age of Trump is child’s play. The Internet’s many ideology-heavy, fact-free silos help too. In various intellectual and moral vacuums, Left and Right, lies about the Jews abound, without colliding and potentially contradicting each other.Bigotry is a social disease, a pathological perception of one group that infests the body politic. Demagogues exploit these social viruses politically. Periodically, when anxiety spreads in society, reasonable arguments and high ideals fail to move enough people. When more constructive arguments fail, when ideals seem tarnished, when cynicism reigns, clever politicians start targeting enemies from within or without.Western demagogues have long enjoyed picking on Jews. Thus, white supremacists fighting blacks add Jew hatred for good measure. And some feminists, while attacking sexists, gratuitously target Zionists too.To the rational mind, throwing in Jews or Zionists looks like mission creep, unnecessarily distracting or even adding potential contradictions. But political bullies recognize the easy mark. Taking the shortcut cultivated by centuries of Western Jew-baiting to recruit more dupes, they bypass the prefontal cortext’s logic center, triggering the amygdala’s aggression center.Tragically, today’s blind partisanship makes life easier for America’s new antisemites. Too many liberals, including Jews, repudiate altright antisemitism while overlooking or even excusing what we can call alt-left or far Left antisemitic anti-Zionism. At the same time, too many conservatives, including Jews, denounce progressive anti-Zionism, caricaturing America’s campuses as anti-Israel war zones, while staying silent about altright antisemitism – and President Donald Trump’s instinct to dodge the fight.These myopic partisans aren’t seeing clearly. They confront those they are least likely to influence. Clear red lines, stretching from Left to Right, should distinguish Jew hatred from legitimate discourse – which of course includes criticism of Israel’s leaders and policies. Liberals should spearhead the fight against left-wing anti-Zionism as conservatives fight right-wing antisemitism.Such consistency and courage within our particular polarized camps, is key to combating all bigotry. More broadly, rather than just fighting the bigotry of those they hate, Americans can defeat this demagogic moment with a new generosity of spirit. Americans should address the underlying anxieties, expose the manipulators, and start dreaming together again. In a long history filled with Jacksonian demagoguery, Civil War era ugliness, Gilded Age manipulations, Depression-era riots, McCarthyite smears, Sixties-era violence, Americans always knew to restore civil discourse, to create a new consensus. And Americans must find leaders who evoke their inner angels, individually and communally – rather than unleashing the devils that lie within us all. The writer is the author of The Age of Clinton: America in the 1990s. His forthcoming book, The Zionist Ideas, which updates Arthur Hertzberg’s classic work, will be published by The Jewish Publication Society in Spring 2018. He is a Distinguished Scholar of North American History at McGill University. Follow on Twitter @GilTroy.