When Parliament votes on Theresa May's Brexit deal on Dec. 11, Britain's future with the European Union may not be the only fate at stake: so too might be that of Ms. May's government. If it falls, Labour leader Jeremy Corbyn could well succeed her as prime minister. And that gives pause to many in Britain's 280,000-strong Jewish community. Under his leadership, Labour has tacked left – and in the process uncorked a torrent of anti-Semitism that Mr. Corbyn, a longtime critic of Israel and campaigner for Palestinian rights, is accused of condoning. Jewish lawmakers and activists in Labour who criticize Corbyn or speak out on anti-Semitism have met with hate speech and death threats. Social media posts by Corbyn’s leftist supporters share “globalist” Jewish conspiracies that parallel far-right propaganda across Europe and the United States. Corbyn denies holding anti-Semitic views, arguing that he stands against all types of racism. But critics say British leftists who pride themselves as being anti-racist suffer from myopia, refracted through the Israeli-Palestinian conflict, that prevents them from equating tropes of Jewish world domination with racism. “People feel that [Corbyn] will not be on our side,” says Adam Langleben of the Jewish Labour Movement. “People feel that racists will be emboldened by his election.”

As Britain enters the endgame of its exit from the European Union, Prime Minister Theresa May is struggling to sell her Brexit deal to a skeptical Parliament. Should it fail to pass, as many predict, the political fallout could end in a snap election.

For the opposition Labour Party, this would be a political gift. It’s running neck-and-neck with Ms. May’s Conservative Party in national polls, as the Brexit debate drives a wedge through her minority center-right government. Labour leader Jeremy Corbyn said last week that his lawmakers would vote against the Brexit deal and that if May loses, the “only sensible course of action” is a general election.

But for Britain’s Jews, the prospect of Mr. Corbyn becoming prime minister is vexing, to say the least.

Under his leadership, Labour has tacked left – and in the process uncorked a torrent of anti-Semitism that Corbyn, a longtime critic of Israel and campaigner for Palestinian rights, is accused of condoning. Jewish lawmakers and activists in Labour who criticize Corbyn or speak out on anti-Semitism have met with hate speech and death threats. Social media posts by Corbyn’s leftist supporters share “globalist” Jewish conspiracies that parallel far-right propaganda across Europe and the US, including among some supporters of President Trump.

This surge in left-wing anti-Semitism, and Corbyn’s fitful attempts to contain it, have rattled Britain’s roughly 280,000 Jews. Nearly 4 in 10 polled recently by a Jewish newspaper here said they would consider exile if Corbyn came to power. In July, that newspaper and two others published a joint front-page warning that a Corbyn-led government would pose an “existential threat” to their community.

While that strikes some Jews as hyperbolic, given Britain’s legal protections and political stability, most agree that Labour harbors virulent anti-Semites in its ranks. “The problem is now embedded at the grassroots level of the party,” says Adam Langleben, a former Labour councilor and an executive of its main Jewish affiliate, the Jewish Labour Movement.

Moreover, many believe that Corbyn, as a politician and an ideologue, is not minded to root out anti-Semitic allies and that their influence is now entrenched in the party’s governance.

“We should be extremely worried. This is an institutionally anti-Semitic party [that] is close to power,” says Euan Phillips, a disgruntled Labour activist whose group, Labour Against Antisemitism, monitors and reports alleged hate speech by members.

Creeping extremism

Corbyn denies holding anti-Semitic views, arguing that he stands against all types of racism. But he has also tried to downplay controversial comments by his allies and claimed that he was being unfairly criticized over what happened in “pockets within the Labour Party.”

Some of the tension with British Jews stems from Corbyn’s decades of opposition to Israel and embrace of its foes. As a Labour backbencher, Corbyn hosted members of Hezbollah and Hamas. In 2014, he attended an event in Tunis at which slain Palestinian militants accused by Israel of the 1972 Munich Olympics terror attack that killed 11 Israelis were commemorated, among others. He has since defended his presence as honoring victims of a separate Israeli air attack. [Editor's note: This original version left unclear that the Tunis event commemorated multiple Palestinians, not just those involved in the Munich attacks.]

Matt Dunham/AP Britain's opposition Labour party leader Jeremy Corbyn arrives to attend a memorial service at St Martin-in-the-Fields church in London in April 2018.

Asked about the concerns of British Jews living under a Corbyn government, a Labour spokesperson said the party “is fully committed to the support, defense, and celebration of the Jewish community and its organizations.”

Last month, police in London said they’d begun a criminal inquiry into hate crimes within Labour. One case involves alleged death threats against Luciana Berger, a Labour member of Parliament who is Jewish. She has complained that Labour didn’t inform her or the police of the threats made against her, which has led to her being given police protection.

The Labour spokesperson insisted the party takes seriously any threat against MPs. “We encourage people to report matters to the police if they suspect a crime has been committed and we expect anyone who has committed a crime to be dealt with,” the spokesperson said.

To be sure, extremism within Labour is not the only threat to Jews and other minorities in Britain. In 2016, Labour MP Jo Cox was murdered during the Brexit referendum by a Nazi sympathizer. British Muslims have also been attacked by far-right vigilantes.

Yet the rise of anti-Semitism within Labour is a reminder that such views have long roosted at political extremes; Nazi Germany and Soviet Russia both propagated myths of Jews as capitalist exploiters who were alien to the European nations in which they lived.

Myopia to anti-Semitism?

The turning point for Labour came in 2016 when Corbyn faced a leadership challenge from MPs who saw him as an electoral liability. When the challenge failed, Corbyn and his allies turned on the rebels, who included Jewish MPs and party donors.

Out came the online trolls alleging a “Jewish conspiracy” to unseat Corbyn. And in came the Trotskyists and other hard-left activists who moved to purge moderates from local Labour groups. “That’s when I realized this was an anti-Semitism issue,” says Mr. Langleben, who was a councilor in Barnet, a district in northwest London where around 1 in 5 British Jews live.

British leftists who pride themselves as being anti-racist and anti-imperialist will share online tropes of Jewish world domination, not equating them to racism, says Langleben. This myopia, refracted through the Israeli-Palestinian conflict, is hard to combat, not least because pro-Corbyn activists distrust mainstream media that criticize him and his politics.

“Anti-Semitism is at its core a conspiracy theory. So if your political movement is prone to not believing the media and believing conspiratorial things, like the Labour Party is right now, it is itself at risk of buying into anti-Semitic myths,” he says.

In May, Langleben ran for reelection in Barnet, which the Conservatives narrowly held. Labour strategists saw the council as an easy target, given the party’s popularity in London. But many Jewish traditional Labour supporters voted Conservative or stayed home. Langeleben and Labour slid to defeat.

He knows why he lost his seat and why he feels lonely at Labour gatherings. “I have no [Jewish] friends left in the party. Everyone has left,” he says.

‘Don't isolate. Engage.’

One prominent Jewish activist who hasn’t given up on Labour is Danny Rich, a rabbi who heads a liberal Jewish movement. In the same elections, he won a seat on the Barnet council, even as Jewish voters told him they couldn’t vote for Corbyn and questioned his allegiances.

“Leaving Labour is not fighting against anti-Semitism. It’s abandoning the fight against anti-Semitism,” he says in an interview at a synagogue in his ward.

Mr. Rich is a pragmatist – and a target of fierce intra-Jewish criticism – who believes that isolating Corbyn is counterproductive. “It’s not in the interests of the Jewish community not to have relations with the leader of the opposition,” he says.

In September, Rich hosted Corbyn and his wife at a Friday Shabbat dinner at his house, along with several Jewish friends and family members. There was no political agenda or debate. “I wanted people to feel comfortable in a religious Jewish home. That was it,” he says.

Earlier that month, Labour agreed to adopt in full an international definition of anti-Semitism after months of infighting over whether its clauses curbed criticism of Israel. For critics of Corbyn, the controversy encapsulated his failure to draw a line between free speech and hate speech.

Rich says Corbyn may have been right to query the precise definition of the International Holocaust Remembrance Alliance. But his dogmatic arguments make for bad politics and make it harder for Labour to win back Jewish voters. “He’s an idealist,” he says.

That Corbyn is so insistent on such issues reflects his conviction that he’s on the right side of racism, even as he defends anti-Semitic speech in Labour, says David Hirsh, a sociologist at Goldsmiths College, University of London, and author of “Contemporary Left Antisemitism.”

Corbyn “believes himself to be a ‘good man,’ ” says Mr. Hirsh. “He believes himself to be an anti-racist man.”

High stakes for Labour, Jews

The electoral cost of Corbyn’s fumbling of anti-Semitism in Labour may be limited to a handful of seats in London and Manchester with large Jewish populations. For most voters, the big issues at the polls will be Corbyn’s left-wing economic agenda and the shifting politics of Brexit. But in a tight general election, a handful of seats could make all the difference.

Hirsh argues that leftist ideology trumps electoral math. “Jeremy Corbyn is wedded to these politics so strongly that he’s willing to put his whole project at risk,” he says.

Mr. Phillips, the disgruntled activist, says that when Labour does investigate alleged anti-Semitism, the offenders are rarely punished for their views. His network of activists have reported over 1,200 individuals in the last two years, mostly for posts on social media.

In March, Christine Shawcroft, the party’s head of discipline, resigned over her defense of Alan Bull, a Labour activist who was seeking a council seat in Peterborough. Mr. Bull had been suspended for posting a link on Facebook to an article denying the Holocaust. Ms. Shawcroft, a close Corbyn ally, wanted him reinstated so he could run for office.

Phillips, who is not Jewish, says Corbyn’s allies invoke free speech and accuse him of silencing debate over Israel. But that defense no longer rings true to him. “There’s a point at which a desire to be open-minded and to be inviting debate moves into moral cowardice and moral bankruptcy,” he says.

While a Corbyn-led government would certainly take a tough line on Israel, how its rule would affect British Jews is unclear. Even his critics concede that government policy is unlikely to change overnight. What they fear is a gradual mainstreaming of anti-Semitic politics in Britain at a time of populist unrest across Western democracies.

“People feel that [Corbyn] will not be on our side. He will not have our back. People feel that racists will be emboldened by his election,” says Langleben.

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Analysts draw comparisons between the mushrooming of hate speech on Labour’s left flank with the racist expressions and violent acts, including anti-Semitic attacks, that have surged in the US since Mr. Trump took power.

“I’m not afraid of Jeremy Corbyn passing laws against Jews or anything like that,” says Hirsh. “The fear is of political people on the left and the right for whom anti-Semitic politics becomes more and more normal.”