The battle of narratives over defeat in the Copeland by-election will determine the future of Labour. For the left, it is both comforting and true that, as in swathes of the north and midlands, Copeland’s steadily declining Labour vote can be traced to the political hollowing out of the party which occurred during the Blair years. New Labour abandoned its working-class base, cosied up to wealthy elites and gave anti-migrant sentiment the space it needed to develop into a full-blown political movement.

Then again, if we are to lay the blame for Copeland at the door of Labour’s past failings, maybe we should talk about the mass disillusionment following the second Harold Wilson government, or James Callaghan’s deals with the International Monetary Fund. Come to think of it, if Ramsay MacDonald hadn’t formed the national government, Labour would undoubtedly be stronger today. Political movements do not get to choose the circumstances under which they challenge for power. The job of any leadership is to win, at the very least on the terms it sets itself, in whichever time it exists.

At the moment, the Corbyn project is not delivering on its promises – and not because it has been entirely unsuccessful. Every poll shows that on the major social and economic issues of the day – ending austerity, public ownership, taxing the rich – the public are supportive of Jeremy Corbyn. Momentum, clipped of its left-wing fringes and now largely a mobilising body rather than a democratic organisation, has the feel of a permanent standing leadership campaign. Together, these facts are a machine for winning unlimited Labour leadership elections.

The problem is that what Corbyn had to do to win Labour is almost antithetical to what can win the country. “Kinder, gentler politics” is motherhood and apple pie to many Labour members, who disdain infighting and aggression. But in an era of hardship, resentment and populism, what the public wants is not “kinder, gentler politics” but “heads on sticks”. Labour is losing by-elections and sitting 16 points behind in the polls because it is failing to present a clear narrative for whose heads should go on which sticks.

Bernie Sanders shocked the world last year by seriously challenging for the Democratic nomination. He did so on a political platform which was decisively to the right of Corbyn, but using rhetoric which sounded, if anything, more left wing. Sanders confronted his audiences with unabashed class politics, hammering the economic elites. Corbyn’s policies are far more radical in substance, but they are delivered in a way which seems more measured – and confused.

From one angle, it is not Corbyn that is making Labour unelectable, but the Labour party which makes Corbyn unelectable. The strength of the Labour Right in the parliamentary party has pushed the leadership to abandon or equivocate over its most radical visions. You could see this process at work as early as October 2015, when John McDonnell was forced to u-turn after endorsing George Osborne’s fiscal charter. The Shadow Cabinet’s positioning on immigration in the wake of Brexit is, truth be told, reminiscent of Ed Miliband’s – accepting that immigration levels have been too high and ditching free movement in principle.

Labour’s leadership fails not when it says what it thinks, but when it engages in a game of chess which it cannot win. If a northern left behind voter wants to end European free movement more than they want cheap high quality housing and higher wages, they aren’t going to vote Labour. For every fair-weather ally and every inch of ground gained inside the Westminster bubble by seemingly clever triangulation, Corbyn compromises the clarity he needs in order to effectively speak to the country.

Labour is an ancient, tight-knit family; its loyalists, who tend to be the most senior people on both wings, have a culture of accommodating one another. The right knows that it cannot survive without the left’s energy, activists and ideas; the left is tempted by the respectability and credibility its policies can derive from Labour’s status as an establishment party. But in reality, this kind of respectability and pre-occupation with holding Labour together is the last thing we need.

When the Copeland byelection was lost, something cracked in Labour’s psyche. Some are already picking over the carcass, seeking to explain the failure of the Corbyn project: was it too left wing, or was it incompetent? For those who want a socialist Labour government, there can be no such council of despair – but we must be unsentimental about where we are.

That could well mean breaking the taboo of contemplating a succession – but much more importantly, it means sharpening our political act. Hurtling towards the edge of an electoral cliff, Labour must go hard or go home.