All over the world anti-Semites are becoming mainstreamed. It is no longer disqualifying to be outed as a Jew hater. This is especially so if the anti-Semite uses the cover of rabid hatred for the nation-state of the Jewish people. These bigots succeed in becoming accepted — even praised — not because of their anti-Semitism but despite it. Increasingly, they are given a pass on their Jew-hatred because those who support them admire or share other aspects of what they represent. This implicit tolerance of anti-Semitism — as long as it comes from someone whose other views are acceptable — represents a dangerous new trend from both the right and left.

In the United States, although there has been hard-right anti-Semitism for decades, the bigotry of the hard-left is far more prevalent and influential on many university campuses. Those on the left who support left-wing anti-Semites try to downplay, ignore or deny that those they support are really anti-Semites. "They are anti-Zionist" is the excuse du jour. Those on the right do essentially the same: "They are nationalists." Neither side would accept such transparent and hollow justifications if the shoe were on the other foot. I believe that when analyzing and exposing these dangerous trends, a single standard of criticism must be directed at each.

Generally speaking, extreme right-wing anti-Semitism continues to be a problem in many parts of Europe and among a relatively small group of "alt-right" Americans. But it also exists among those who self-identify as run-of-the-mill conservatives. Consider, for example, former presidential candidate and Reagan staffer Pat Buchanan.

The list of Buchanan's anti-Jewish bigotry is exhaustive. Over the years, he has consistently blamed Jews for wide-ranging societal and political problems. In his criticism of the Iraq War, for example, Buchanan infamously quipped: "There are only two groups that are beating the drums for war in the Middle East — the Israeli Defense Ministry and its amen corner in the United States." He then singled out for rebuke only Jewish political figures and commentators such as Henry Kissinger, Charles Krauthammer and A.M. Rosenthal. Buchanan did not mention any of the vocal non-Jewish supporters of the war. Furthermore, Buchanan also said that "the Israeli lobby" would be responsible if former President Barack Obama decided to strike Iran, threatening that if it were to happen, "Netanyahu and his amen corner in Congress" would face "backlash worldwide." Buchanan's sordid flirtation with Nazi revisionism is also well documented.

On university campuses, the absurd concept of " intersectionality" — which has become a code word for anti-Semitism — is dominating discussions and actions by the hard-left. The warm embrace of Palestinian-American activist Linda Sarsour — who recently delivered the commencement address at a City University of New York graduation — is a case in point. A co-organizer of the Women's March on Washington in January, she has said that feminism and Zionism are incompatible, stating: "You either stand up for the rights of all women, including Palestinians, or none. There's just no way around it." And when speaking about two leading female anti-Islamists, Brigitte Gabriel and Ayaan Hirsi Ali (who is a victim of female genital mutilation) the feminist du jour, Linda Sarsour, said: "I wish I could take away their vaginas."

The irony is breathtaking. Under her own all-or-nothing criteria, Sarsour, who is also a staunch supporter of trying to destroy Israel economically, cannot be pro-Palestinian and a feminist because the Palestinian Authority and Hamas subjugate women and treat gays far worse than Israel does.

Sarsour supports Islamic religious law, Sharia. If taken literally, this would presumably mean that she also supports punishing homosexuality by death; amputation for theft; death by stoning for "adultery" (which can include being raped); and women's being valued at half the worth of a man, being flogged for drinking alcohol, and above all subjected to slavery (see here, here and here).

Yet Sarsour has emerged as a champion of the hard-left. Both New York City Mayor Bill De Blasio and Sen. Bernie Sanders have sought her endorsement. Moreover, Deputy Democratic National Committee Chairman Keith Ellison, who himself has a sordid history with anti-Semitism stemming from his association with Louis Farrakhan (who publicly boasted about his own Jew hatred), has come out in support of the bigoted Sarsour. When it comes to Ellison, an old idiom comes to mind: A man is known by the company he keeps.

The same trend is detectable among the hard-left in Europe, particularly in Britain, which is days away from an election. The British Labour Party has now been hijacked by radical extremists on the left and is known for being soft on anti-Semitism.

In a recent interview with a BBC reporter, Emma Barnett, who happens to be Jewish, Labour Party leader Jeremy Corbyn fumbled when answering a question about how much his proposed childcare policy would cost. Rather than critique Corbyn, Labour supporters viciously trolled the Jewish BBC reporter. Tweets such as these abounded: "Allegations have surfaced that @Emmabarnett is a Zionist" and "Zionist Emma Barnett (family lived off brothels) attacks Jeremy Corbyn." Corbyn has also been accused of anti-Jewish bigotry himself. He has said in the past that the genocidal Hamas terrorist group should be removed from the U.K.'s designated terror list and has called Hezbollah and Hamas (which are both vowed to the destruction of the nation-state of the Jewish people) "my friends." (I recently wrote extensively on Corbyn's association with some of Britain's most notorious Holocaust-deniers and anti-Semites.)

Increasingly, anti-Semitic discourse is also seeping into the arts and academia. Consider the anti-Israel and anti-Jewish bigotry of former Pink Floyd front man Roger Waters. A staunch supporter of the so-called BDS movement, Waters has said about the Palestinians that "parallels with what went on in the 30's in Germany are so crushingly obvious." He also had a pig-shaped balloon with a Star of David on it at one of his concerts. And when asked about his aggressive effort to recruit people to join the BDS, Waters blamed "the Jewish lobby," which he explained is "extraordinary powerful here and particularly in the industry that I work in, the music industry." In 2013, the ADL declared that "anti-Semitic conspiracy theories" had "seeped into the totality" of Waters' views.

Likewise, the marketplace of ideas on university campuses and within academic institutions has seen an embrace of anti-Semitism often disguised as anti-Zionism. Several years ago, I identified the dangerous trend of academics crossing a red line between acceptable criticism of Israel and legitimizing Jew-hatred. This was in light of the disgraceful endorsement by a number of prominent academics of an anti-Semitic book written by Gilad Atzmon — a notorious Jew-hater who denies the Holocaust and attributed widespread economic troubles to a "Zio-punch."

When asked recently about the hullabaloo surrounding her CUNY address, Linda Sarsour disingenuously played the victim card:

" ... since the Women's March on Washington, once the right-wing saw a very prominent Muslim-American woman in a hijab who was a Palestinian who was resonating with a community in a very large way, they made it their mission to do everything they can to take my platform away."

No, Ms. Sarsour, you are wrong. This is not a smear campaign by the "right-wing" but rather a show that people of goodwill reject your manifestations of bigotry.

Those who tolerate anti-Semitism from those they otherwise admire would never accept other forms of bigotry, such as racism, sexism or homophobia. It's difficult to imagine Bernie Sanders campaigning for a socialist who didn't like black people or who was against gay marriage. But he is comfortable campaigning for Jeremy Corbyn, who has made a career out of condemning Zionists — by which he means Jews.

The growing tolerance for anti-Semitism by both the extreme left and right is quickly becoming mainstream. That is why it is so dangerous and must be exposed for what it is: complicity in and encouragement of the oldest form of bigotry. Shame on those who tolerate anti-Semitism when it comes from their side of the political spectrum.

People on both sides of the aisle must have the same zero tolerance for anti-Semitism as they do for sexism, racism and homophobia. Decent people everywhere — Jews and non-Jews — must condemn with equal vigor all manifestations of bigotry whether they emanate from the hard alt-right or hard alt-left. I will continue to judge individuals on the basis of their own statements and actions, regardless of which side of the aisle they come from.

Alan Dershowitz (@AlanDersh) is the Felix Frankfurter Professor of Law, Emeritus, at Harvard Law School and author of "Taking the Stand: My Life in the Law" and "Electile Dysfunction: A Guide for the Unaroused Voter." This article was previously published by the Gatestone Institute.

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