In 2015, Britain’s Labour Party tacked to the left, repudiating the middle-way philosophy that had won it three elections under Tony Blair. Voters responded by handing the party its worst defeat in three decades.

Rather than scramble back toward the center, Labour lurched further left. The party elected as its leader Jeremy Corbyn, a white-bearded baby boomer from the back benches who, like Bernie Sanders in the United States, ignited an improbable movement among young activists with his attacks on the rigged capitalist system and unquestioned fidelity to socialist ideals.

Now, with less than six weeks to go before Britain votes once more, the Corbyn-led Labour Party is on course for an electoral beatdown so broad and deep it would make the drubbing the party took in 2015 look like a triumph.

The ruling Conservative Party has a double-digit lead over Labour in pre-election polls, and Prime Minister Theresa May stands to win a parliamentary majority that would have been the envy of Margaret Thatcher. 

The grim outlook for Labour has prompted insiders to preemptively concede defeat; one former party leader has despaired that at 75, he’s unlikely to see another Labour prime minister in his lifetime. There’s even a chance that the party could fall apart altogether.

Prime Minister Theresa May, shown leaving leaves the BBC, in London, April 30, 2017, has gone on the attack even in working-class northern English constituencies that haven’t voted Conservative in decades. (Peter Nicholls/Reuters)

The decline of Labour — architect of the country’s vaunted National Health Service and one of two major parties in Britain for the past century — offers a cautionary tale for Democrats as they attempt to rebound from a humiliating 2016 loss to Donald Trump.

Corbyn may have captured the hearts of left-wing true believers. But unless something dramatic changes before June 8, when Britain votes, that’s not enough to win a national election.

“He’s still very popular with a lot of Labour activists. But he’s a long way from the center of gravity among the British people,” said Martin Baxter, a political analyst who runs Britain’s Electoral Calculus website. “The lesson for Democratic voters who thought that Bernie Sanders would revitalize the party is that in Britain at least, with Jeremy Corbyn, it’s not worked out.” 

Nor has it worked out in other places where center-left parties have attempted to placate their increasingly radicalized grass roots by shifting toward the margins. 

In France, for instance, the Socialist Party — beleaguered after a humbling five years in power marked by double-digit unemployment and a slew of terrorist attacks — rejected more centrist alternatives and picked as its presidential nominee Benoît Hamon, a proud radical who championed a 32-hour workweek and a universal basic income.

But in the first round of the vote on April 23, Hamon mustered only an embarrassing 6 percent, having split the far-left vote with a Socialist defector, Jean-Luc Mélenchon, who proposed nationalizing France’s biggest banks and withdrawing from NATO. Neither made the final round.  

Meanwhile, 39-year-old Emmanuel Macron, a former economy minister under the incumbent Socialist government, ran to the center via his own upstart movement that aims to combine strands from both the left and right. He is now the favorite to become the next president of France.

The Socialists, said French political analyst Gérard Grunberg, suffered for too long under “an establishment that’s aging, tired and not active in modern communication.” 

Now the election outcome has left in doubt the future of the party, which helped build one of postwar Western Europe’s most generous welfare states. 

Britain’s Labour Party is facing its own existential angst even before the country goes to vote.

“Labour is the party of the industrial proletariat — that was its original function. But Britain doesn’t have an industrial proletariat anymore,” Baxter said of a party that traces its roots to 1900 and the workers’ rights movements of factory-saturated northern England. “So there’s a big question as to what the Labour Party is for.” 

Corbyn has sought to offer an answer by promising a more “socially just society.” That involves passionate opposition to the austerity policies enacted by the Conservatives since they came to power in 2010, ending 13 years of unbroken Labour rule. 

The 67-year-old has pledged to halt the cuts in public services, to renationalize banks and energy firms and to consider a “maximum wage” on private-sector executives. Corbyn has also been highly critical of NATO, the British nuclear deterrent and the European Union — though he grudgingly backed “remain” in last year’s Brexit vote.

To his enthusiastic backers — who delivered the north London lawmaker a pair of landslide victories in party leadership races — Corbyn’s prescription for Britain is exactly what the country needs.

“I love him — best leader ever,” said Richard Crook, a 57-year-old telephone engineer from southeast London who cheered Corbyn on at the lawmaker’s campaign kickoff. “We’ve all had enough of PR politics. We want the truth. He is a bit like Bernie Sanders. He’s leading us into a fight back.” 

Such fervent support helps explain why Corbyn has insisted he does not believe the polls. 

“I’m out on the streets and the doorsteps and the meeting halls every day, and that’s not what I’m finding,” he said during a campaign stop last week.

But there’s no getting around the fact that the polls for Labour are dire. The Tories now have a working majority of 17 in the 650-member House of Commons. Projections — which no doubt influenced May’s decision to call the snap vote — show that could widen to 150 or more. 

The gains are forecast across the U.K. 

In Wales, where the Conservatives haven’t won in nearly a century, a recent survey showed them leading. In Scotland, where Labour ran a virtual one-party fiefdom until the 2015 vote, the party is now a distant third.

May has gone on the attack even in working-class northern English constituencies that haven’t voted Conservative in decades. And she’s doing so by invoking Corbyn at every turn. 

“I know this city is one of the places that people call a ‘traditional Labour area’,” May said at a Thursday night rally in Leeds, a Yorkshire city that was once renowned for its wool mills. “But here — and in every constituency across the country — it may say Labour on the ballot, but it’s Jeremy Corbyn that gets the vote.”

The strategy is not hard to understand: Polls show that fewer than half of Labour’s own voters favor Corbyn in a head-to-head matchup with May, and his broader approval ratings are abysmal. His rigidly leftist views and reputation for incompetent management help explain why.

“He seems like a throwback to the 1970s,” said Tim Bale, a politics professor at Queen Mary University of London. “He’s not someone voters have warmed to. They neither like nor respect him. Indeed, he seems a figure of ridicule.” 

Blair, the last Labour prime minister to win a national vote but a reviled figure among the party’s leftist grass roots, is among those who have declined to endorse Corbyn. In an interview with Britain’s Sky News last week, he said the identity of the prime minister after the June election is no mystery: “It’ll be Theresa May.” 

That’s because even as Corbyn has energized some voters, he’s alienated many more. 

“The man is living in a cloud cuckoo world,” said Gareth Bell, a 34-year-old business development manager and, until now, Labour voter.

Bell, an ardent pro-European who recently defected to the centrist Liberal Democrats, said he was disenchanted by Corbyn’s muddling stance on Brexit. Five of his friends have also left the party, he said, “and the ones that are still there are holding on by their fingertips. The moderates are so disappointed in him.”

If Labour does lose in a rout, Corbyn may be forced to resign as party leader. The party could also split apart.

Whether that happens or not, the center-left the world over will have to work out what it stands for and stop re-litigating internal battles that date back decades, said Stewart Wood, a Labour member of the House of Lords and top adviser to former party leader Ed Miliband.  

“The center-left is struggling everywhere with a philosophical malaise,” he said. “It has to be about the next 20 years, not the last 30. Any party that’s busy trying to pick its favorite moment from the past is in trouble.” 

James McAuley in Paris contributed to this report.