Mr. Corbyn’s long history of slurs against Jews and Israel, his warm words about his “friends” in Hezbollah, and his worldview, which finds far more to admire in countries like Russia and Venezuela than England itself, are well documented. To choose just the latest headline: The BBC, he said on Iranian state television, is “biased” toward saying that “Israel has a right to exist.” (My favorite remains his remark that British Zionists “who, having lived in this country for a very long time, probably all their lives, don’t understand English irony.”)

The fish rots from the head, and so it has with Labour. According to Britain’s chief rabbi, Ephraim Mirvis, there are 130 cases of outstanding complaints of anti-Semitism against Labour Party members. Ninety-three percent of British Jews say they won’t vote for Labour. Forty-seven percent say they will “seriously consider” emigrating if Labour wins. And yet the latest polling shows Labour rallying.

Other issues, Jews are told, are more important than their own safety. Sometimes their fears are dismissed as “hysteria.” The socialist filmmaker Ken Loach has called it a “witch hunt.” The powerful union leader Len McCluskey has accused the Jewish community of “intransigent hostility.” When Rabbi Mirvis took the extraordinary step of weighing in on the election, insisting that the very “soul of our nation is at stake,” he was accused, variously, of bad faith, of taking focus away from the real threat of right-wing bigotry, and of actually stoking anti-Semitism himself.

Jewish Voice for Labour, the party’s pet Jewish front group, has codified all of these gaslighting tactics in a document that helpfully lays out the strategy for how other political groups and movements with a will to power can shut down Jewish concerns about organized, systemic anti-Semitism.

Over in Italy, the town of Schio decided against establishing a Holocaust memorial — the subtle, brass “stumbling stones” called stolpersteine that dot the streets of European cities where survivors of Hitler’s genocide still stroll — on the grounds that 14 stones in a city of 40,000 would prove too divisive. “Let the victims rest in peace” said Alberto Bertoldo, a local politician. The memorial, he said, would risk “generating new hatred and division.”

While in Schio the Holocaust proves a divisive moral issue, in Montreal it was a planned trip to Israel. Jordyn Wright is a Jewish sophomore who sits on the board of the Students’ Society of McGill University. Over winter break, she is planning, like hundreds of other North American Jewish college students, to go to Israel with Hillel. As a result of that trip, the student government voted to call for her resignation. Never mind that the trip included time with Palestinians in the West Bank. Never mind that another student government leader is also going; apparently because that student is not a Jew, no resignation was required.

I toggled over the weekend between Ms. Wright’s chilling account of the history of anti-Jewish discrimination at her school and The Washington Post, which had published a frothy profile of Valerie Plame, the former C.I.A. officer who is now running for Congress as a Democrat in New Mexico.