During the improbable summer of 2015, when Jeremy Corbyn went from being an unknown, sixty-six-year-old leftist Member of Parliament to the leader of Britain’s Labour Party, there was a natural urge to know more about him. Journalists and bloggers, supporters and skeptics, all picked over Corbyn’s thirty-two-year parliamentary career, reading old speeches and looking into the causes he had adopted and the company that he kept. There was plenty to go through. Ever since he was elected as a Labour councillor for Haringey, in North London, in 1974, and later, as the M.P. for Islington North, in 1983, Corbyn has been the kind of politician who shows up to a pro-Sandinista rally on his bicycle, stays late in the House of Commons to protest the removal of Tamil asylum seekers, or sits through a sleepy Saturday conference about abolishing nuclear weapons.

Corbyn has long campaigned for peace in the Middle East, and he has frequently criticized the actions of Israel. Over the years, he has attended protests and conferences alongside campaigners who have expressed anti-Semitic views. In mid-August, 2015, a month before Corbyn was elected Labour leader, Stephen Pollard, the editor of the Jewish Chronicle, which describes itself as the oldest continuously published Jewish newspaper in the world, wrote a front-page editorial challenging the candidate to explain some of these instances. “They were all things in the public domain. We weren’t revealing anything new,” Pollard told me the other day. “But nobody had really paid attention to Corbyn previously, because why would you?” Pollard wrote the editorial while on holiday in Devon. Headlined “The Key Questions He Must Answer,” it asked Corbyn about his connections to Deir Yassin Remembered, an anti-Israel group run by a Holocaust denier; his defense of an Anglican vicar who peddled anti-Semitic conspiracy theories; and his descriptions of Hamas and Hezbollah as “friends,” and of Sheikh Raed Salah, a Palestinian mayor accused of making the blood libel in 2007, as “an honored citizen.” “I just sat down in my cottage and wrote this leader,” Pollard said. “That set the course for the next couple of years, really.”

Allegations of anti-Semitism—committed by members, officials, and the leader himself—have been the running sore of Corbyn’s leftist takeover of the Labour Party ever since, and the sense of something gravely wrong has deepened with time. This summer, with Theresa May’s Conservative government perilously weakened by Brexit, Corbyn is closer than ever to becoming Prime Minister. But he is also involved in an unprecedented confrontation with the main institutions of Britain’s Jewish community, led by the Chronicle, the Jewish Leadership Council, and the Board of Deputies of British Jews, whose history of representing Jewish people dates back to the reign of King George III. On July 25th, Britain’s three leading Jewish newspapers published a joint article on their respective front pages, warning of “the existential threat to Jewish life in this country that would be posed by a Jeremy Corbyn-led government.” When I asked Pollard what that meant, he replied, “They wouldn’t set up camps or anything like that. But the tenor of public life would be unbearable because the very people who are the enemy of Jews, as it were, the anti-Semites, will be empowered by having their allies in government. There is a fear, a real fear of that.”

To Corbyn’s supporters, the idea that he is hostile to Jewish people is a low attack and an absurdity. Anti-racism and inclusiveness are organizing principles of Corbyn’s politics, so the notion that he could be anti-Semitic—or could allow prejudice to go unchecked—is a kind of logical fallacy. “He is a prominent campaigner for human rights, quite without malice,” a spokesman said, in response to the Jewish Chronicle’s editorial, in 2015. “He does not have an anti-Semitic bone in his body.” To Corbyn’s critics inside and outside Labour, however, this is what happens when a form of fringe leftism—in which a loathing of Israel and of global capitalism has been known to morph into Jew hatred—enters the political mainstream. Either way, it is a terrible situation for a party that has been the natural home for most British Jews for the past hundred years. “Jews have no better friends in this country than the Labour Party,” the Jewish Chronicle reported, in 1920. As recently as 2014, Corbyn’s predecessor, Ed Miliband, another trenchant critic of Israel, spoke of his dream of becoming Britain’s first Jewish Prime Minister.

Each of Corbyn’s attempts to respond to the issue has somehow managed to make things worse. In the spring of 2016, when I was reporting a Profile of the Labour leader for this magazine, Naz Shah, a member of Corbyn’s Shadow Cabinet, was suspended from the Party for sharing Facebook posts that suggested that Israelis should be relocated to the U.S. (“Problem solved,” she wrote.) The following day, Ken Livingstone, a former mayor of London and long-term Corbyn ally, went on the radio to defend Shah and talked about Hitler instead: “He was supporting Zionism before he went mad and ended up killing six million Jews.” Livingstone was suspended as well. In response, Corbyn ordered a two-month inquiry into anti-Semitism in the Labour Party by Shami Chakrabarti, a respected civil-rights lawyer.

Chakrabarti’s forty-one-page report concluded that the Labour Party was “not overrun” by anti-Semitism, but noted an “occasionally toxic atmosphere” inside the Party. It recommended that members not use slurs such as “Zio,” or “Hitler, Nazi and Holocaust metaphors, distortions and comparisons,” when talking about Israel and the Palestinians. Chakrabarti also called for a sense of proportion: inquiries of this kind should not automatically be reduced to a “witch-hunt” or a “white-wash.” She did not get her wish. At the press conference to announce the findings of the Chakrabarti report, in June, 2016, a pro-Corbyn activist accused Ruth Smeeth, a Jewish M.P. sitting in the audience, of working “hand in hand” with a right-wing newspaper to undermine the Party. Attacked with an anti-Semitic trope at an event intended to draw a line under anti-Semitism in the Party, Smeeth left the room in tears. Two months later, Chakrabarti accepted a peerage from Corbyn. The Board of Deputies of British Jews described her report as a “white-wash.”

That was Act I. Act II took off a few weeks ago, when the Labour Party was faced with the question of whether to adopt an internationally agreed-upon definition of anti-Semitism. The text, which is suggested by the International Holocaust Remembrance Alliance, is clunky: it contains a definition, followed by eleven examples of anti-Semitism in public life. The wording is ungrammatical and occasionally combines ideas that might otherwise have been kept apart. The seventh example reads, “Denying the Jewish people their right to self-determination, e.g., by claiming that the existence of a State of Israel is a racist endeavor.” The tenth: “Drawing comparisons of contemporary Israeli policy to that of the Nazis.” You can argue about whether it is accurate or constructive to describe the state of Israel as racist or to compare the I.D.F. to the Wehrmacht, but you can also argue about whether those criticisms are inherently anti-Semitic. (In October, 2016, a British Parliamentary committee suggested adding two caveats to the definition, to protect freedom of speech about Israel.)

Despite its deficiencies, the I.H.R.A. definition has been adopted by the European Parliament, the U.S. Senate, and eight national governments, including Britain’s. Given the Party’s past three years, under Corbyn, and the ragged nature of its relationship with Britain’s Jewish leaders, people expected the Labour Party to do the same. Ahead of the decision, sixty-eight rabbis wrote an open letter urging Labour to adopt the text. The U.K’s chief rabbi, Ephraim Mirvis, a genial, non-political figure, described the vote as a “watershed moment.”