As news came in of Labour’s loss in Copeland, an argument started between two wings of the party. Jeremy Corbyn’s supporters used Twitter to blame Tony Blair for its decline among its northern working-class base, while Blair’s supporters argued that Corbyn was culpable. The polling says they are both right.

Labour’s collapse among working-class voters is catastrophic – according to YouGov, only 16% would vote Labour now. That’s troubling enough for “the party of working people”, but it is made doubly damaging because, contrary to expectations, Ukip is not proving the main beneficiary. These voters are increasingly voting Conservative. After seven years of Tory austerity, Labour is 15 points behind the Tories among working-class likely voters, having been ahead in 2015.

While the proportion of the population that is working-class is falling steadily, it remains hard to see a route to power for Labour if it cannot secure a majority in this group.

Over the last year I have been running focus groups for various parts of the Labour movement with warehouse pickers, scaffolders, care workers and check-out assistants. The disdain they have for Corbyn is remarkable. There is no sense that he is on their side, or has any of the capabilities they expect of a prime minister.

As one woman from Rochdale put it, he “should be sat on a barge somewhere floating up and down”. Corbyn is now 36 points behind Theresa May as the preferred prime minister among working-class voters. There is no way back from this position. Corbyn believes he is standing up for working people’s interests, but they don’t buy it. Someone is suffering from false consciousness, but it’s not clear it’s the voters.

However, it would be a profound misreading to think he is the sole cause of the problem. Its roots run much deeper. New Labour had tremendous success winning support across the social classes in 1997, but in government it was much better at hanging on to its middle-class voters than the less well-off. Between 1997 and 2010, for every voter Labour lost from the professional classes it lost three unskilled or unemployed workers, even after taking into account the declining share of the population that pollsters classify as working-class.

This was not a coincidence. The party’s strategy was to take core supporters for granted while courting the middle classes. The one time in the run-up to the 2005 election that I was asked by the party to run focus groups with the less well-off, their view of Labour was so devastating that the decision was taken never to talk to them again. The target audience continued to be “soccer moms” (sic).

As a result, the party lost focus on the issues that these voters care about. Top of the list was immigration, where the party got tough with refugees while ignoring the ultimately much bigger issue of low-skill, high-volume, short-term immigration. Labour came to be seen as a party that put migrants before British citizens.

Gordon Brown’s “bigot” remarks about Gillian Duffy in Rochdale nearly prompted the party to confront this problem, but it proved too hard to get anything done. The proportion of working-class voters seriously concerned about Labour on immigration was consistently more than 60% throughout the last parliament – higher than concerns about handling the economy.

Corbyn inherited a brand that had been steadily alienating its working- class base, and he made it worse.

The nightmare for Labour is that May has clocked the opportunity to win these voters. Where David Cameron’s huskies-and-windfarms modernisation was aimed at the chattering classes, May’s modernisation aims to show the Tories aren’t just a party of toffs. Her rhetoric is succeeding, marginalising Ukip and threatening catastrophe for Labour.

James Morris, senior director at the public relations firm Edelman, is a former Labour pollster