Jeremy Corbyn always had a soft spot for the Soviet Union.

In 1986, he criticized the British press for feeding people “pro-American propaganda... and a diet of anti-Soviet propaganda.” In 1989, he called for the “complete rehabilitation” of Leon Trotsky in the British Parliament. In 1991, he said: “I am concerned at the breakup of the Soviet Union and the leadership it gave and the breakup of the Socialist International.”

This man is favorite to become Britain’s next prime minister.

And month by month, he is reshaping the Labour party in his image. Much of the world still associates Britain’s Labour with Tony Blair—the slick, centrist, and interventionist prime minister who stood shoulder to shoulder with George W. Bush as the U.K. and U.S. invaded Afghanistan and Iraq in the years following the Sept. 11, 2001, attacks.

But in Corbyn’s three decades in the British Parliament, he’s voted against every major Western intervention, labeled NATO “the father of the Cold War” and called for its disbandment. As you may have noticed, these positions were also championed by the Kremlin.

Corbyn admitted that in the 1980s he met with a Czech spy who described him as “a very, very good source”—although there’s no documented evidence that Corbyn was ever a spy or agent of influence, as some of the more excitable British tabloids and politicians have claimed.

There is no denying, however, that the current Labour leader’s attitude to the Soviet Union and Russia has long been much more sympathetic than that of most of his party colleagues. That became a major problem this week, in the wake of the attempted murder of former spy Sergei Skripal with one of Russia’s homemade chemical weapons.

Corbyn was first muted in his response, and then appeared reluctant to hold the Kremlin responsible when he was forced to address the brazen attack on a British citizen.

His closest adviser, the former Guardian columnist Seumus Milne, has a backlog of op-eds backing Russia over the West. He even sat on a panel with Russian President Vladimir Putin.

That undercurrent of anti-Western sentiment within Corbyn’s inner circle burst into public view this week.

The House of Commons—which often sounds like feeding time in a farmyard—fell silent Wednesday in a moment of rare consensus when Prime Minister Theresa May announced that 23 “undeclared intelligence officers” (in other words, Russian spies) would be thrown out of the country as punishment for the suspected state-sponsored assassination attempt on Skripal.

The attack on the former spy and his daughter, Yulia, has stoked fury and condemnation toward Russia from around the world, and most of Britain’s political class was unanimous that action had to be taken against the Kremlin. That consensus was punctured when Corbyn took to his feet to respond.

The Labour leader did join in the condemnation of the attack, calling it “an appalling act of violence,” describing the use of a poisonous nerve agent as “abominable” and saying it was “utterly reckless to use them in a civilian environment.” But he then decided to leave open the possibility—eliminated by the British government and later by the leaders of France, the U.S., and Germany—that a party other than the Russian state was responsible for the attack. He failed to directly criticize the Kremlin for it.

Corbyn’s statement—which attracted cries of “shame” from the government and was criticized, directly and indirectly, by many of his own party members—was later compounded by Milne, the spokesman he had elevated from a small band of hard left commentators. Milne suggested Corbyn lacked faith in the government’s decision to blame Russia because of the dodgy intelligence on WMDs before the Iraq War.

Milne told journalists: “I think obviously the government has access to information and intelligence on this matter which others don’t; however, also there’s a history in relation to WMD and intelligence which is problematic to put it mildly.”

He went on to suggest alternative culprits to the Russian state, echoing the Kremlin’s explanations for the attack, saying: “We highlighted today and we have done repeatedly, the dangers of mafia-like groups and oligarchic interests in London, and their links with elements within the Russian state, and that we need to take more firm action on that.”

Milne’s words—which were so remarkable that the British press broke with the standard convention of anonymizing spokesmen and women to call him out specifically—were met with an equally furious reaction by the governing Conservative party and his own colleagues in Labour.

Labour Member of Parliament Ian Murray told The Daily Beast: “The fact that a chemical weapon has been used on mainland Britain against British citizens should shake everyone in the international community. That is why there has been such anger that [Corbyn], and the favorite to be the next prime minister, sought to use the time in Parliament to ask the questions of the PM that the Russians were posing.

“The evidence is clear that the only possible explanation as to whom is responsible is the Russian state either directly or indirectly. For Jeremy Corbyn’s adviser to suggest to the press that this is Iraq all over again is deplorable. Nothing is more important than the safety of the public and everyone across every single political party in the House of Commons backed the prime minister’s response—apart from [Corbyn] and his spokesperson.”

Murray added: “He was right to say we need to keep some dialogue open with the Russians, but he was wrong to merely dismiss the evidence from the security services. Let’s not forget he gets access to highly classified information and has chosen to ignore it.”

Labour MP Chuka Umunna, once tipped as a future leader, said: “Milne’s comments do not represent the views of the majority of our voters, members, or MPs. We’ll get abuse for saying so, but where British lives have been put at risk it is important to be clear.”

Veteran Labour MP Mike Gapes, added to the criticism, saying: “I understand that Jeremy Corbyn’s spokesman, Seumas Milne, has claimed there is no proof that Russia is behind the nerve-gas attack in Salisbury. Well Seumas has form on these matters. Not in my name.”

Despite the attacks from his own party, Corbyn doubled down on his position Thursday, saying May was rushing ahead of the evidence. “Labour is of course no supporter of the Putin regime, its conservative authoritarianism, abuse of human rights, or political and economic corruption,” he said. “However, that does not mean we should resign ourselves to a ‘new Cold War’ of escalating arms spending, proxy conflicts across the globe, and a McCarthyite intolerance of dissent.”

The bitter infighting is a callback to the first years of Corbyn’s leadership, which was largely papered over when Labour did better than expected in the 2017 snap general election and prevented May’s Conservatives from having an overall majority in Parliament. Corbyn, initially considered a radically left-wing outsider in the party, used to be routinely criticized by his own colleagues who felt he and his allies were pushing the party too far to the left.

The tensions between Corbyn supporters and the more moderate Labour groups—which were so spectacularly ripped back open this week—have rarely been about domestic issues about which most in the broadly social democratic Labour party agree. It’s foreign policy—specifically Corbyn’s past relationships with groups such as the IRA and Hamas, the Labour left’s apparent problems with Israel, the Iraq War, and Russia—that have stoked up most of the warring words between Labour’s groups.

What this week’s infighting has exposed yet again is the fight for the soul of the Labour party and, crucially, how it will interact with the rest of the world should Corbyn win power. One recent opinion poll showed Labour with a 7 percentage-point lead over May’s Conservatives—though others have May marginally ahead—and the party has stayed on election footing since last year’s general election in a hope that government divisions over the exit from the European Union will eventually cause a collapse.

Since Corbyn became leader in 2015, there has been a concerted effort to create a radically different Labour party to match the leader’s particular strain of politics. A pro-Corbyn pressure group called Momentum has ballooned in size and influence within the party, Corbyn allies now hold the majority of seats on Labour’s powerful ruling body, and this year has started to see accusations of the party deselecting moderates in favor of so-called Corbynites. The deselection process allows local Labour party groups to replace the incumbent with a more radical alternative in time for the next election.

The leader of a London council claimed she was forced out by left-wing bullies earlier this year, saying she was targeted by Momentum with “sexism, bullying, and undemocratic” behavior. More than 20 center-left councilors in the area were deselected or decided not to stand again, with Momentum candidates taking their places. The episode stoked fears that similar deselections could be brought in at the next general election to remove lawmakers who don’t fully back Corbyn.

While Momentum has denied reports there’s a “hit-list” of up to 50 Labour members of parliament that could be replaced by pro-Corbyn candidates for the next election, its founder reiterated the belief that activists could take it upon themselves to replace politicians who fail to “listen to their members,” which some interpreted as a dog whistle instigating deselections.

A prominent British journalist close to Corbyn, Paul Mason, has also previously backed the idea of deselection, saying: “Just like when we go to the coffee shop and get to choose between latte, cappuccino, and tea, I want once in every five years to have it hanging over every lawmaker that they could be deselected if they don’t do their job properly.”

The threat is so real to some Labour members of parliament that some have even threatened to jump before they’re pushed in deselection battles by quitting and standing as independent candidates.

Not only is the once-laughable idea of Corbyn becoming prime minister—this time last year he had a lower popularity rating than Donald Trump among the British public—now entirely possible, but if he and his allies do succeed in creating an ultra-loyal left-wing party behind him he’d be free to enact exactly the kind of policies at home and abroad that he wants under Britain’s democratic system, which grants a prime minister with a clear majority in parliament almost unlimited political power.

If Corbyn becomes Britain’s prime minister, the world will be introduced to a leader who could scarcely be more different to Tony Blair. As this week has shown, he would join President Trump in rejecting the Western postwar consensus and leave us wondering: Is there anyone left to stand up to Putin?