At no point in post-World War II British history has anti-Semitism seen the prominence in national politics that it has in the lead-up to Thursday’s general election. The British Labour Party under Jeremy Corbyn is now widely considered to be, in the words of some of its own former members of Parliament, “institutionally anti-Semitic,” thrusting the issue of anti-Semitism into the spotlight and leaving Jews questioning their future if the party wins.

The specter of anti-Semitism shrouds Corbyn so thickly that nearly half of U.K. Jews told pollsters they would seriously consider leaving the country if he becomes prime minister. This response comes despite generations of Jews feeling secure and at home in Britain under Labour as well as Conservative governments.

In part, this is a story of a political accident, in which the Labour Party fell unexpectedly under the control of the radical left and its standard-bearer Jeremy Corbyn.

Moreover, even if Corbyn fails, as seems likely, his leadership has unleashed previously unthinkable attitudes on the British left that have made Jews deeply uncomfortable — and those odious sentiments and the unease they cause will not easily dissipate.

How is it that anti-Semitic attitudes have suddenly become such a prominent political issue in Britain? Did Britain suddenly become anti-Semitic? Far from it. Anti-Defamation League surveys indexed anti-Semitic attitudes in Britain at just 11 percent in 2019, close to the U.S. at 10 percent and lower than France or Germany. Over the last two decades, there have been more Israelis settling in London than British Jews leaving.

So how is Labour embroiled in anti-Semitism? In part, this is a story of a political accident, in which the Labour Party fell unexpectedly under the control of the radical left and its standard-bearer Jeremy Corbyn, who for more than 30 years was a mostly irrelevant member of Parliament on Labour’s far-left margins. Grey, aging, bearded and unfashionable, few outside parliament or the party had heard of him, and few who knew of him paid him attention.

But when Labour held a party leadership election in 2015 after successive general election defeats, Corbyn caught a wave. He used activist networks developed in the 2003 anti-Iraq War campaign and benefited from short-sighted rule changes introduced by his predecessor to energize the party by allowing nonparty members to vote. His tirades against injustice and promises of radical change carried appealing authenticity, including for younger activists.

So where do Jews come in? For many, this crumpled old lefty — who likes to speak of his lifelong opposition to racism — could not look less anti-Semitic. Jew hatred conjures images of far right attacks in the U.S. or Islamist animosity fueled by religious extremism.

Thankfully, far-right anti-Semitism has long been fairly marginal in mainstream British politics. And the European radical right tries to exploit antipathy toward Muslims more than Jews nowadays. When it comes to British Muslims, anti-Semitic attitudes are more prevalent among them than the general population, but that has not significantly resonated with the dominant culture of either mainstream political party.

The left, however, harbors its own long-standing anti-Semitic traditions. In the 19th century, Jews were identified with international capitalism and finance; during the Cold War, Soviet propaganda tied these old myths to Zionism (the movement that created a Jewish nation-state in Israel, which almost all British Jews support).

Accordingly, many radical left activists in the U.K. — such as Corbyn, who became politically active in the late 1960s — saw Israel as a racist colonizer to be opposed along with the rest of Western imperial power, rather than an expression of Jewish self-determination and answer to persecution. Pro-Palestinian activism has long been a badge of identity for the left of the party, in contrast to the warmth towards Israel shown by centrist Labour leaders such as Tony Blair and Gordon Brown.

Corbyn has a long record of associating with those expressing not mere criticism of Israel, which can be perfectly acceptable, but hatred of Israel and Jews, and even Holocaust denial. One infamous video shows the British politician describing Hamas and Hezbollah as “friends” working for “long-term peace,” though both are openly committed to the destruction of Israel and murder of Jews, and proscribed as terror organizations by the U.S. and the U.K.

In another, Corbyn reveals his own deep prejudices when he refers to “Zionists” who “having lived in this country for a very long time, probably all their lives, don’t understand English irony.” The implication is unmistakable: Even if born in England, Jews are not really English.

And he has allowed anti-Semitism by others in the party to go unchecked under his leadership. In 2016, for instance, former London Mayor Ken Livingstone repeated debunked accusations of collusion between Zionists and Nazis prior to World War II. These remarks would previously have been condemned without reservation, but Corbyn failed to insist on expelling Livingstone, signaling a permissive environment for such views.

Even more troubling is that the public outcry about Corbyn’s sentiments has led his supporters to dig in. Many Labour activists have come to regard charges of anti-Semitism made by the Jewish community as part of a politically motivated smear to discredit their leader.

That has contributed to normalizing appalling ideas among sections of the Labour ranks, and to a confusing public debate about what constitutes anti-Semitism. Since Corbyn became leader, examples of anti-Zionist and anti-Semitic hate speech of the grossest form have ballooned among leftist activists, especially on social media, while many of those calling out anti-Semitism, Jewish and non-Jewish, have left the party in despair.

Meanwhile, measures within Labour to expel offenders have been half-hearted. While Corbyn in 2018 said he was sorry for the hurt caused to many Jews, party whistleblowers claimed processes to expel offenders were held up. In a high-profile BBC election interview in November, Corbyn declined an invitation to apologize to the Jewish community.

Now the fear is a Corbyn-led government would not only be deeply hostile towards Israel, but list the Jewish community among its enemies, threatening both community morale and interests such as government support on security for Jewish institutions and Holocaust education.

The good news for British Jews and their many sympathizers across a wide spectrum of the media and politics is that Corbyn is unpopular. Though anti-Semitism is not a primary concern for most voters, it adds to Corbyn’s liabilities and widespread doubts about his suitability.

In a social media age of conspiracy theories on steroids, anti-Semitism — the most enduring of conspiracy theories — is surging on the British left.

However, even if Corbyn loses and is ultimately replaced, Labour has been transformed under his leadership: Its radical left wing now dominates the party machinery, significant numbers of moderates have left, and anti-Semitic attitudes have been unleashed. Some of his devotees will surely blame any loss on a conspiracy by the media, financial interests and, yes, British Jews.

In a social media age of conspiracy theories on steroids, anti-Semitism — the most enduring of conspiracy theories — is surging on the British left. British Jews can only hope that Corbyn will soon be replaced by someone less personally afflicted with anti-Semitic prejudices, and more capable of rooting out the poisonous strain of thinking that now infects the party.