Jeremy Corbyn is running out of excuses. Losing a seat that has been held by Labour in every election since 1935 certainly signifies a break from the old politics, but not the one that was advertised to Labour members.

The explicit promise of Corbyn’s leadership campaigns was reconnection with the party’s founding spirit and values, leading to a recovery of votes in places that had drifted away from Labour under Tony Blair, Gordon Brown and Ed Miliband. But Miliband’s Labour party held Copeland.

Byelections are unreliable guides to future general election outcomes but effective for capturing moments of electoral volatility. Traditionally they give voters an opportunity to lash out at an incumbent government to the benefit of the opposition. It is rare for that dynamic to be reversed. Under the conventional rules of politics, Labour would be shovelling votes on to an increased majority in a seat like Copeland.

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The threat of closure hanging over a local hospital maternity unit furnished just the kind of local issue to propel a lively mid-term swing against a government. When Labour can’t even mobilise its core supporters – or rather, those voters who once constituted its core – in defence of the NHS (and that was the focus of the party’s campaign on the ground) something has gone very wrong.

Or someone. Campaigners for all parties report that Corbyn’s name was coming up on the doorstep. His well-known aversion to nuclear energy did not go down well in a seat where Sellafield employs 10,000 people. The Tories were not shy of reminding people that the Labour leader was ideologically hostile to the engine of their local economy. But MPs who canvassed the constituency report a deeper frustration with Corbyn – a sense that he simply isn’t up to the job; that he has been miscast in a role to which he isn’t suited; that the party is insulting its longest-serving supporters by telling them that this man should be their prime minister when they can see that he wouldn’t be able to do the job and might not even really want it.

The same message was communicated by voters in Stoke-on-Trent Central, although Labour held off a Ukip challenge there. No one who canvassed that seat believes the result contains an endorsement of Corbyn. It would have been inconceivable for a Stoke Central contest to be even marginal until recently. The corrosion of support for Labour in this notional “heartland” area cannot be attributed to the party’s current leadership. It is the expression of a social, economic and cultural dislocation that has been building for many years. But Corbyn’s leadership was meant to repair that damage. It was sold as the tonic to restore faith in socialism where New Labour had whittled that faith away.

The decisive factors in Stoke appear to have been residual, tribal Labour loyalty – in spite of the current leadership – plus an electoral machine that is a bit rusty but still good for identifying and turning out the most loyal voters.

Most important was the good fortune to have, in Paul Nuttall, a Ukip challenger who radiated dishonesty and opportunism. Labour canvassers working the constituency yesterday were heartened to discover a firewall of anti-Ukip feeling aiding their cause. Nuttall plainly thought he could turn up from the outside, march around the Potteries blowing his Brexit trumpet like some tweed-clad Eurosceptic Joshua and see the walls of a Labour Jericho crumble.

He was wrong. Ukip were lucky to beat the Tories in this contest which has proved, yet again, that the party is not good at winning Westminster seats. It lacks a slick machine for bringing dedicated activists to do the unglamorous work of pavement-pounding politics, and is generally struggling to find a purpose now that its founding purpose – quitting the EU – has been adopted wholesale by the Conservative party. And even then, Ukip’s vote share went up and Labour’s fell.

That is hardly a great consolation for the leader. As one MP who was knocking on doors in Stoke reports: “Corbyn is a massive issue. Every single activist has a horror story to tell about what they’ve heard people say about him … It’s not sustainable.”

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There is a rebuttal to that testimony. If Corbyn’s supporters dig really deep into their faith, they can say that Labour MPs have a jaundiced view, that they are unreliable witnesses because many of them tried to oust their leader last year, and that in fact it was their treachery that destabilised the whole project. Opinion polls might show the Tories with double-digit leads over Labour, but maybe perfidious parliamentarians should take the blame for that. And the media, of course. There is always the option of blaming a partisan press for the non-contagion of Corbyn’s message.

It is true that a divided party whose MPs have bellowed out loud their lack of confidence in the leader will struggle to make electoral headway. It is also true that some British newspapers write about politicians of the left with vindictive aggression. There is ample responsibility for Labour’s problems to go around – it needn’t all collect in a puddle at Corbyn’s feet. And Theresa May can take some credit for her own relative popularity, too. She must be doing some things right for Copeland to swing into Tory arms.

The demoralisation of core Labour voters in Cumbria and Staffordshire predate the current leadership. But the current leader was chosen as the antidote to decline, not its amplifier. And when canvassers meet ex-Labour voters who now abstain or have switched to the Tories, it is not turbulent MPs’ names that come up as the reason. The parliamentary rebellion was crushed last summer. There is little internal impediment to the communication of the leader’s message now.

If he is in possession of an inspiring vision and an evangelical gift that can stir a mass movement to sweep Labour back to power, there really isn’t that much stopping him from deploying those gifts. The airwaves are available; so is the vast expanse of social media and any number of venues willing to host another rally. That was the plan when Corbyn sought ownership of the party. And now he owns it. So must he also own last night’s failure.