At the last general election, 9.3 million people voted Labour. That number should represent the foundation on which a successful challenge might be built for next time. It looks more like a resource for other parties to plunder. According to one recent poll, only 69% of those who last year endorsed Ed Miliband back Labour now.

In a byelection in the Lincolnshire seat of Sleaford and North Hykeham last week, Labour slipped from second to fourth place. In Richmond Park the previous week, the Labour candidate lost his deposit.

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Jeremy Corbyn’s leadership campaigns promised to create a dynamic social movement, shoring up traditional support while mobilising battalions of eager converts. Instead, Britain’s main opposition party resembles a dilapidated warehouse storing heaps of votes behind rusted gates, guarded by a drowsy night watchman. With a modicum of ingenuity, other parties can break in and help themselves.

It would be unfair to heap all of the blame on Corbyn. Those of us who were hostile to his candidacy from the start have to accept that our instincts were wildly out of tune with the mood among a majority of Labour members. There was no prospect of a revival down the route of haughtily dismissing their beliefs as naive or plain wrong. In the summer of 2015, Corbyn himself was taken aback by the surge in support for his message. Forces bigger than any individual, forces that have broken timid centre-left politics across Europe, were manifest in his landslide victory.

Corbyn’s critics were especially slow on the uptake in thinking that the vagueness of his programme, the primary-coloured sketch of back-to-basics socialism, was self-evidently a problem. For an older generation of left stalwarts and a new generation of radical dreamers the problem was just as self-evidently located in the world-weary, eye-rolling arrogance of the New Labour ancien regime, which insisted the choice be limited to a menu of uninspiring compromises.

There is no obvious leader-in-waiting who might sustain the exuberance of the Corbyn revolution among the faithful while winning the trust of less engaged voters unmoved by the radical spirit. While the Labour “moderates” acknowledge the paltriness of their platform, many of the leader’s backers are quietly realising the limits to his magnetism. It attracts too few voters in seats that are needed to win a majority, and repels many more. Labour’s privately commissioned research tells the same story as the published polling: failure to make advances among Tory voters; vulnerability to Ukip in areas that strongly rejected Britain’s EU membership, and to a Lib Dem recovery among those who most grieve at the referendum result; a flatline in Scotland.

Facebook Twitter Pinterest ‘Owen Smith’s incoherent challenge proved that Corbyn thrives on attacks from within his party.’ Photograph: Robert Perry/EPA

One point of consensus is that the second leadership contest, this summer, was a waste of everyone’s time. Owen Smith’s incoherent challenge proved that Corbyn thrives on attacks from within his party. He cannot be accused of lacking potency in an internal campaign, but it is a defensive kind of charisma. It is as if he wears a protective shell made from reflective material that looks more brilliant in the eyes of his fans when a harsh light is trained on it. When the political beam falls elsewhere – on the Tories and Brexit – Corbyn is invisible. He seems to have no inner source of radiant energy.

Corbyn’s supporters complain that he is traduced by partisan media and that his poor ratings express the fallout from months of sniping by backbench MPs. But the parliamentary party has been relatively well-behaved since September’s re-coronation and difficulty turning press releases into favourable news stories is hardly a new conundrum for Labour.

In conversation with Corbyn’s allies I have been struck by how much their dilemmas replay problems confronted by Miliband’s team. They know there is an issue with the leader’s image. He is not taken seriously as a potential prime minister. They know they need a strategy for turning things around but they can’t say what it looks like.

They hope voters can be persuaded to embrace an unorthodox concept of leadership. They highlight the way their candidate connects with individuals on the campaign trail. They propose that Labour’s offer to run the country be considered as a collective endeavour, not some glitzy presidential bid. But they don’t know how to make that happen.

The last line of defence is that recent electoral upsets – the EU referendum, Donald Trump’s US victory – prove the fallibility of conventional wisdom (and polling). Perhaps the discontent with mainstream politics could be channelled into a renaissance for the radical left. But again, there is no known method for effecting this diversion. In the absence of a plan, the belief that an upswing in reactionary nativism is a veiled opportunity for Corbynism, with its embrace of open borders and awkwardness around symbols of national identity, is fanciful to say the least. It sounds like self-congratulation for disbelieving incorrect forecasts of rain, then proudly stepping into a hailstorm without an umbrella.

Labour is marooned between a discredited past and an unimagined future. Many of the obstacles to recovery predate the current leader. The difficulty in articulating a clear response to Brexit, for example, stems from the absence of common instincts on the best approach to immigration, free trade, markets, and on protecting people from the economic vagaries of globalisation without retreating into bitter rejection of the modern world.

Labour is marooned between a discredited past and an unimagined future

Miliband grappled unsuccessfully with that conundrum. So now does Theresa May. Corbyn seems untroubled by it. His position as a moral figurehead on the left, representing defiance of an intellectually bankrupt old guard, is fixed. Those who disagree have learned that resistance without a credible counter-offer makes their position worse. But even the leader’s allies see the gap between his symbolic status and the lack of practical ability to move the project forwards. He is neither setting a direction nor resolving dilemmas.

So a slow-motion race is under way for the succession involving pretty much any frontbencher with a high profile – John McDonnell, Clive Lewis, Keir Starmer, Emily Thornberry among others. None dares eye the crown in public. All recognise that the contest, when it comes, will probably be won by someone who can show the membership a record of loyalty – of having at least tried to make things work under Corbyn.

Meanwhile, the incumbent gets to be just “Jeremy” – a rallying cry, an icon, a standard to hold aloft when enemies approach. He has won the battle to decide what Labour no longer wants to be, but he has no answers to the question of what it does next. He is unassailable at the top of the party, but he is not leading it.

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