What happens when the evidence starts piling up that the leader of a major political party is an anti-Semite? Do his party’s supporters force him out of office? Or do they rally around him, claiming that his political agenda is too important to be sacrificed for the sake of a marginal issue and constituency, and instead seek to blame the Jews and other people of conscience speaking about it for causing trouble?

No, I’m not talking about U.S. President Donald Trump. Whatever his other failings, the arguments put forward by many Jews who oppose his administration that he hates Jews are utterly unpersuasive. That’s especially true when you consider his close family connections and the fact that his policies have positioned him as the best friend Israel has had in the White House in decades.

But the same can’t be said about Jeremy Corbyn, the man who may be Britain’s next prime minister. The head of the United Kingdom’s Labour Party, Corbyn has long a record of hostility to Israel and Jewish causes, as well as support for local radicals who hate Jews, in addition to the Hamas and Hezbollah terrorist groups. But in recent weeks, the list of Corbyn’s outrages has grown.

After tolerating open anti-Semites in his party, Corbyn’s supporters sought to embrace a definition of anti-Semitism that would still allow them to compare Israel to the Nazis, as well as oppose its right to exist with a clean conscience. When faced with Jewish complaints, his backers arrogantly declared that the Jews were the ones with the problem.

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That was enough to motivate Britain’s three leading Jewish newspapers to each publish a front-page editorial blasting the “Corbynite contempt for Israel and Jews,” stating that a Britain led by Corbyn would constitute an “existential threat to Jewish life” in the country.

If that wasn’t bad enough, since then evidence has surfaced proving that Corbyn honored the perpetrators of the 1972 Munich Olympic massacre at their graves in Tunisia. He’s also now been shown to be a good friend of a Holocaust-denier .

When Israeli Prime Minister Benjamin Netanyahu condemned Corbyn’s laying a wreath on the graves of terrorists who murdered and mutilated 11 Israeli athletes, the Labour leader responded by not only issuing absurd denials of a documented event, but doubling by down on his smears of Israel. He accused Netanyahu and the Israel Defense Forces of killing “dozens” of Palestinian children—a factually incorrect reference to the casualties resulting from Hamas’s assaults on Israel’s Gaza border during their near-weekly “Marches of Return” since March 30. Not only is Corbyn on record honoring terrorist murderers, he has also managed to align himself again with an effort aimed at destroying the Jewish state.

But while there’s little doubt that Corbyn is a habitual Jew-hater, many Americans are probably asking why they should care about him at all.

The growing popularity of Corbyn’s radical socialist agenda and the fact that the governing Conservative Party is on the verge of a split over Brexit have made a Labour victory at the next election a real possibility.

The United Kingdom he hopes to lead is but a shadow of the great power it once was. Though Britain remains culturally important, its say in world affairs is minimal. But the problem here goes beyond the possibility that a country with a permanent seat on the U.N. Security Council would be led by a man with such deep-seated hostility to Israel and Jews.

Corbyn’s rise is significant because it demonstrates the way the European left has slid into a position of comfort with open anti-Semitism. More than that, it sets a precedent that American progressives need to worry about as the Democratic Party in the United States may be taking a hard left turn.

Comparisons between the Democrats and Labour—or any European party of the left—are facile and often misleading.

Though Labour went through a modernizing period under Tony Blair, it has now reverted to its traditional support for socialism and other radical causes. By contrast, the modern Democratic Party is liberal, but capitalist and centrist by European standards. The question about the Democrats now is whether, in the wake of Hillary Clinton’s presidential defeat, it is about to take a decisive step to the left.

While the extent of their influence is debatable, there’s no question that two avowed socialists—Vermont Sen. Bernie Sanders and New York contender for a House seat Alexandria Ocasio-Cortez—are being treated as the rock stars of the party. That shows the appeal of the left for those who style themselves as progressives. It also shows the power of intersectional ideology—in which the struggles against prejudice at home are seen as linked to those of Third World peoples against the West—which, to one extent or another people like Sanders, Ocasio-Cortez and many of those organizing the anti-Trump “resistance” have embraced.

For anyone who succumbs to the appeal of intersectionality, Israel is a villain. That assertion goes a long way towards understanding why someone a progressive like Corbyn makes common cause with so many Jew-haters, while denying rights to Jews that he would never think of refusing to any other people. While criticism of Israel isn’t necessarily anti-Semitic, opposing Israel’s right to exist or to defend itself is clearly defined as anti-Semitism. Yet when faced with complaints about Corbyn’s anti-Semitic tendencies time after time, most of the British left has circled the wagons around him and isolated the Jewish community.

Even worse, if he achieves power, Corbyn will be seen as the model for left-wing politicians around the world, including Americans who long for a challenger to Trump who will be as brutal to their opponents as the president.

It’s doubtful that Democrats would nominate anyone as radical as Corbyn in the foreseeable future. Even though he’s similar in many ways to Sanders, Corbyn is much more extreme. But if the party continues to drift away from a position of support for Israel as the stands of their two Socialist rock stars and other prominent left-wingers indicates, that means progressives must be on their guard to avoid falling into the same ideological trap as the Brits.

Jonathan S. Tobin is editor in chief of JNS — Jewish News Syndicate. Follow him on Twitter at: @jonathans_tobin.