On June 9, 1983, buried in the news of the landslide victory that returned Margaret Thatcher to 10 Downing Street for a second term, Jeremy Corbyn was elected to Parliament. Outside the confines of the tight circle of far-left activists with whom he had spent most of his adult life, Corbyn was largely unknown and would remain so through most of the next three decades until his shock election as leader of the Labour Party in September 2015.

Even so, on that June day, one Jewish member of the local party in north London, which had chosen Corbyn as its candidate, had already seen enough. Philip Kleinman, a columnist for the Jewish Chronicle, published an article in the Evening Standard saying that, thanks to Corbyn’s backing for policies that would effectively mean the “destruction of Israel,” he would not be voting Labour. Kleinman was expelled as a member by Labour. He wouldn’t be the last Jew forced from the party due to Corbyn and his agenda.

Corbyn’s sudden rise to the leadership didn’t require him to shed his anti-Zionist ideological clothes. He wore them proudly. More remarkable still is the fact that in under four years, they’ve become the garb of his party—a party British Jews once considered their natural home. Corbyn’s leadership and the party’s transformation in his image has caused a mass exodus of Jewish members and supporters from its ranks and has split the party asunder.

Why should Americans pay attention? As Senator Bernie Sanders releases videos labelling Gaza a prison camp and, together with some of his fellow aspirants for the Democratic Party’s presidential nomination, shies away from denouncing those in their ranks who propagate nakedly anti-Semitic tropes, some in America have detected the creeping onset of Corbynism. They are right to do so.

Now, there are huge cultural, historical, and institutional differences between Labour and the Democrats—as, indeed, there are between the British and American Jewish communities. But the example of Labour’s descent into the sewer of anti-Semitism is nonetheless an instructive and cautionary tale. It is a story of how seemingly marginal figures with views and values far removed from the party’s mainstream were at first ignored, then tolerated, and, finally, legitimized, with devastating consequences.

Corbyn’s anti-Zionist views and activities have defined much of his political life. Throughout the 1980s, he was a supporter of the Labour Movement Campaign for Palestine and regularly spoke at its events. The group supported Israel’s replacement by a democratic, secular Palestinian state and pledged to “eradicate Zionism” from the party. Reviewing Corbyn’s overall record shortly before he became leader, the Jewish Chronicle pointed to “overwhelming evidence of his association with, support for…Holocaust deniers, terrorists, and some outright anti-Semites.”

Few mainstream Britons paid any attention to Corbyn, his Marxist ideology, or activism. He rarely featured in the media (beyond the pages of the pro-Communist Morning Star, for which he wrote frequently), and, as David Kogan writes in Protest and Power, his newly published account of the party’s recent history, he appeared to operate “not off the radar but under the radar.” The truth, Kogan adds, is that during this period, “no one in the leadership really cared what Jeremy Corbyn was doing.”

Corbyn was viewed as so marginal and semi-detached that, when he publicly described Hamas and Hezbollah as “friends” in 2009, there was little attention, let alone repercussions. It’s not as though Corbyn was doing pernicious things in secret. Revelations of anti-Semitic connections and virulently anti-Israel comments have dogged his leadership, but they stem from articles he wrote, the many appearances he made on Iran’s Press TV, and his speeches at demonstrations in the center of London. Even Corbyn’s sharpest critics wouldn’t charge him with attempting to hide his views or tailor his message to different audiences.

In 2017, a senior Labour Party official revealed that Tony Blair—whose “New Labour” movement” dragged Labour to the center-left in the early 1990s—had actually vetoed efforts by moderates in Corbyn’s constituency to deselect him as their candidate during his premiership. Despite the rebellious backbencher’s repeatedly voting against the government that Blair led, the prime minister simply couldn’t conceive of Corbyn as any kind of threat.

Corbyn might have remained the obscure figure on the Labour backbenches he was for his first 32 years in Parliament were it not for a major change in 2014 to the Labour Party’s rules for choosing its leader. Ironically, given their ultimate outcome, these were driven through by the party’s first Jewish leader, Ed Miliband. An adviser to, and later minister in, the Blair-Brown governments, Miliband was elected as the party’s leader following its ejection from office in 2010.

Indeed, many moderates supported Miliband’s introduction of a form of primary in which members of the public could register as party supporters and then vote in leadership elections. This was seen as a way to balance the powerful role played by the increasingly left-leaning trade unions. But Miliband’s plan went much further. Members of Parliament, who had effectively acted as “superdelegates” able to cast one-third of the votes in party-leadership elections, found their role virtually eliminated. This meant that a crucial check on the left’s sway among the grassroots and trade unions was lost. Although it was by no means clear at the time, the path to the leadership for a hard-left candidate suddenly became much easier.

Like Sanders, the aging Corbyn was able to win the backing of many young people. But behind this idealistic youthful cohort, Corbyn was also mobilizing a powerful coalition of older, hard-left activists. Many of them had previously had nothing to do with Labour—or had been expelled from the party in the 1980s—and had instead been involved in tiny, highly sectarian and factional Marxist and Trotskyite parties. Many others had also been active in groups such as the anti-American Stop the War Coalition and the Palestine Solidarity Campaign, in both of which Corbyn had long played a leading part. Under Labour’s new rules, and without the veto of the parliamentary party, this alliance swept Corbyn to an unexpected victory. Together with some powerful trade-union muscle exerted on his behalf, it has subsequently proved crucial in helping Corbyn weather the political storms that he has endured over the past four years, not least a leadership challenge and a vote of no confidence by fellow MPs.

But organizational changes weren’t the only helping hand that Corbyn was to receive from Miliband. During the five years he led the party, Miliband also laid the ideological groundwork for Corbyn. He signalled the change in his first speech as party leader. “The era of New Labour has passed,” Miliband boldly declared, before launching into a denunciation of the Iraq war. That headline-grabbing pronouncement from the leader of the party that had taken the country to war in 2003 was not the only sign that Miliband intended to shift Labour to the left. Moments later, he abandoned the pro-Israel line that had characterized the premiership of Blair and Brown, and he shifted wholesale into the anti-Israel camp.

“The Gaza blockade must be lifted and we must strain every sinew to work to make that happen,” he pronounced. Now, criticizing aspects of Israeli policy in the manner Miliband did is not in itself anti-Semitic. But in retrospect, this was the moment when Labour’s current negative obsession with the Jewish state began. For in his 6,000-word speech, Miliband did not address any other foreign-policy issue beyond Israel and the war in Iraq.

Miliband’s regular attacks on much of the domestic and foreign-policy record of the government of which he had been a member helped give credence to Corbyn’s later rhetorical assault on the Blair years as a triumph for neoliberal economics at home and American-led military adventurism overseas. That assault opened the door to the new leadership’s own hard-left alternative agenda and also cast those associated with the former government as complicit in this apparently shocking enterprise.

During the 2014 Gaza war, which was begun by Hamas with rocket attacks on civilian targets, the party strenuously attacked the “killing of hundreds of innocent Palestinian civilians,” which Miliband deemed “unacceptable and unjustifiable.” Anti-Semitic attacks in the UK doubled in the wake of the conflict. And the Labour leadership maintained a studied silence about that for four months. In a further harbinger of the future, Labour then opted to doubledown on its strategy. While anger at its actions simmered among many Jews, the party announced that it would support unilateral recognition of a Palestinian state, backing a motion proposed by a virulent critic of Israel who, just weeks before, had compared the Israel Defense Forces to the Islamic State.

Labour’s anti-Blair lurch to the left set the ideological stage for Corbyn, just as its misconceived populist plan to expand the leadership vote set the political stage for him. When the party elected a new leader in September 2015, Corbyn won nearly 60 percent of the 422,000 votes cast. (Under the old rules, Miliband narrowly beat his brother, David, by 1.4 percent of the overall vote; indeed, David, who was closely associated with Blair, won the votes of 54 percent of the roughly 127,000 members who cast ballots, but that lead was overhauled by union voters.)

Corbyn terrifies Labour members of Parliament, but there’s little they can do about him; he is now in complete control of the levers of party power outside the House of Commons. He has a majority on Labour’s governing body, the National Executive Committee, which plays a key role in setting and interpreting the party’s rules, as well at times in the selection of parliamentary candidates. He has purged Labour’s London headquarters of veteran staff whose loyalty to the new leadership was suspect. His allies control key trade unions that, thanks to their financial contributions and their power within the party’s internal structures, wield huge influence. Finally, and crucially, Corbyn continues to enjoy wide support among the party’s grassroots membership, much of which has joined since his election.

Labour’s unexpectedly strong showing in the 2017 general election pretty much silenced the MPs who would prefer anyone else to be party leader. Secure in his position, Corbyn evinces a haughty disdain for most of his fellow elected members, regularly steering clear of their weekly meetings and refusing to answer their questions when he deigns to make an appearance before them.

The “Corbynization” of the party is evident in the manner in which the boundaries of debate within the party, and its agenda for office, have been redefined. Nationalization and higher taxes—taboo during the years in which Blair governed, followed briefly by his frenemy Gordon Brown—are back on the table. Union demands are invariably accepted. On foreign policy, the deep unpopularity of Donald Trump among the British public has provided cover for a break from the Atlanticism of Blair and Brown. The actions of Russia, Iran, Cuba, or Venezuela are far less likely to be the object of Labour’s frequent outbursts of righteous anger than those of the United States. This was most evident when Corbyn repeatedly refused to condemn Russia following the March 2018 chemical attack by two Russian intelligence officers on a former double agent in the city of Salisbury in southwest England.

No such reticence or call for dialogue—a favorite Corbyn demand—is forthcoming when it comes to the actions of the Middle East’s sole democracy. After the violence on the Gaza border in May 2018, for instance, the party condemned Israel’s “brutal, lethal, and utterly unjustified actions” and accused it of an “apparently systemic and deliberate policy of killing and maiming.” It also blithely endorsed “the right to return to their homes” of the “peaceful protesters” attempting to cross the border from Gaza into Israel. Even rote condemnations of Hamas are now routinely absent from Labour’s statements about Gaza. Compare that with the nuance the party leadership displays when discussing Syria. In October 2016, as public concern about the plight of civilians in Syria mounted, a spokesperson for Corbyn declared: “The focus on Russian atrocities or Syrian army atrocities … sometimes diverts attention from other atrocities that are taking place.” Unsurprisingly, when Hamas recently thanked Corbyn for “showing solidarity with the Palestinian people,” the party did nothing to repudiate this endorsement or distance itself from it.

On these matters, it appears the Labour leadership is in step with its base. Polling released last year at the height of the row over anti-Semitism in the party showed that nearly 80 percent of Labour members believe that accusations of Jew-hate were being exaggerated to damage Corbyn and stifle legitimate criticism of Israel. A similar figure approved of the Labour leader’s job performance, with 61 percent saying he was handling the anti-Semitism crisis well. Polling also revealed the extent of opposition to Israel among Labour’s grassroots. Nearly two-thirds of members said they believe the Jewish state to be “a force for bad” in the world—six points higher than the 59 percent who thought the same of Iran.

So how has Corbyn achieved Labour’s transformation from the moderate, social-democratic party of Blair and Brown into a bastion of the hard left?

First, Corbyn has out-organized his opponents. Shortly after his leadership victory, Jon Lansman, a veteran Jewish hard-left organizer who had run the Corbyn campaign, formed Momentum. Boasting a network of more than 40,000 members and 200,000 supporters in local groups across the country, this parallel pseudo-party acts as the Labour leader’s praetorian guard.

When Labour MPs attempted to oust their leader in the summer of 2016, Momentum mobilized its grassroots activists to ensure his reelection. Social-media savvy, it also played a prominent role in boosting Labour’s fortunes in the 2017 election (thus, once again, helping to save Corbyn’s job). Momentum perfectly captures the blend of politics that has helped entrench Corbynism in the party. On the one hand, it is a campaigning group with a youthful image pushing a radical agenda. Billing itself as a social movement, it is currently calling for Labour to adopt a raft of “transformational” policies in its next manifesto, including a “green New Deal,” the abolition of migrant detention centers, and the introduction of a four-day week. Momentum’s activities have been inspired by Sanders’s 2016 campaign, and it has close links with senior figures involved in it, as well as ties to Democratic Socialists of America.

On the other hand, Momentum also has the mindset and ruthlessness of the old hard left, focusing heavily on changing the party’s rules to make it easier for pro-Corbyn members to oust incumbent moderate Labour MPs. It is also thinking ahead: Last year it secured changes that will make it easier for a hard-left candidate to stand in any future leadership contest. And Momentum isn’t afraid to let its activists loose on Labour MPs who it believes aren’t toeing the Corbyn line; in 2015, for instance, it urged them to lobby those parliamentarians who were thinking of backing the Cameron’s government’s support for the U.S.-led operation against the Islamic State in Syria.

The Corbynites have been boosted by, and in turn helped fuel, the growth and popularity of a raft of digital platforms that constitute an informal network designed to counter the influence of the “old media system,” with its alleged right-wing bias. These include Novara Media—sometimes dubbed Momentum’s “armed police”—and The Canary. This “alternative” media allows Corbyn to bypass the hated Fleet Street press and BBC and communicate directly with his supporters. It has also enabled the Labour leadership to amplify the voices of fringe groups helpful to its cause. Throughout the anti-Semitism debate, for instance, Corbyn has leaned heavily on the backing of Jewish Voice for Labour. A small group of largely anti-Zionist Jews, unrepresentative of the Jewish community and hostile to Labour’s official Jewish affiliate, the Jewish Labour Movement, JVL has defended Corbyn relentlessly and attacked his opponents.

Corbyn has also benefited from the mistakes of his internal critics. Their most fundamental error was to define their opposition to him primarily in terms of his supposed lack of electability. This made them seem both unprincipled and calculating. More important, the strategy was seen to be undermined when, against all expectations, Labour managed to fight the Tories to a virtual draw in 2017.

Finally, Corbyn’s own political and personal attributes must be acknowledged. He has formed, in terms of British politics, a perhaps unique bond with his supporters (cult-like, charge his critics). He has shown a dogged self-belief, and he is driven by an ideological zeal that is often wrongly compared to Thatcher’s (in office, she was much more cautious and pragmatic). When he overwhelmingly lost a vote of no confidence by his MPs in June 2016, he didn’t blink but took up the gauntlet they had thrown down and won a second leadership election with an enhanced majority.

Even in the face of huge political controversy, he often stubbornly refuses to cave in. Last summer, following weeks of negative headlines about anti-Semitism, Corbyn resisted Labour’s adopting in full the International Holocaust Remembrance Alliance definition of anti-Semitism. Instead, he pressed for an amendment saying that it should not be considered anti-Semitic to describe Israel, its policies, and the circumstances of its foundation as racist. Backed by his social-media cheerleaders, such tactics have helped to shift Labour toward more radical positions. Unlike many political leaders, Corbyn thus serves as his own ideological outrider.

The impact of this is evident in the manner in which—even among the small number of Labour MPs who are still willing to speak out against anti-Semitism in its ranks—there is a wariness about addressing questions around anti-Zionism, despite the fact that, on the hard left, the two are inextricably linked.

Wresting Labour back from the grip of the hard left may now be impossible or, at least, a task that will take years to accomplish. The Democrats, however, still have an opportunity to learn from it.

First, ignoring or appeasing the views of those who appear to be fringe figures is a dangerous risk. In uncertain political times, and with the opportunities provided by the “alternative” media, marginal individuals can—if they go unchallenged—take their ideas mainstream with far-reaching consequences.

Second, disregarding the hard, unglamorous and largely thankless task of political organizing comes at a huge cost, as Labour moderates have discovered. The hard left’s relentless focus on connecting and mobilizing their supporters appeared as eccentric as their views during the Blair years. And yet, within five years of Labour’s losing office, that focus played a critical part in Corbyn’s election. At the same time, organization alone is not enough. The lack, among Labour moderates, of an appealing message, their overly technocratic language, and their perceived caution and dearth of ambition and passion helped turn a sexagenarian leftist with an agenda rooted in the 1970s into the hero of young idealists.

Third, any critique of the far left or objections to it should be rooted in values and principles and should not be focused solely on electability. Such a critique can, as happened to Corbyn’s critics after 2017, fall apart all too quickly.

Labour has lost, and seems not to care that it has, most of its traditional Jewish supporters. In the 2017 election, 69 percent of Britain’s Jews voted for the Conservative Party—a stunning number, considering that Labour was the same kind of home for Jews that the Democratic Party has been in the United States. The open anti-Semitism of so many Corbynites—paralleled this year by the noxious words of superstar freshman Congresswomen Rashida Tlaib and Ilhan Omar—have compelled many left-wing Jews (and non-Jews) to abandon their long-time political home. And they have led many others to fear the consequences of it coming to power. The Democrats may be far from that stage, but they stand at a crossroads if they do not act to end the onward march of American Corbynization.