It’s a strange experience to become enemy number one for Britain’s biggest trade union: the machine can strike at any time. At 10:30 last Sunday morning, Unite’s general secretary declared, unprompted, on The Andrew Marr Show, that I had “lost my marbles” over Brexit and should “stop putting pressure” on Jeremy Corbyn.

I fear Len McCluskey’s annoyance will intensify when those fighting to commit Labour to an unambiguous Remain position succeed. He and his allies have turned support for Brexit into an existential issue. For them — as Unite official Howard Beckett put it in a recent New Statesman piece — without a commitment to delivering Brexit, Labour becomes “Corbyn without Corbynism”.

And that is what, for many of us on the Labour left, this argument is really about: who gets to define what a left-wing Labour Party stands for? Five hundred thousand members, or a few officials at Unite’s HQ and their protégés in Corbyn’s office?

Labour was right, after the 2017 general election, to respect the referendum result. There was no concrete Tory Brexit plan laid out; there was every prospect of negotiating a Norway-style deal; and the toxic xenophobia of the 2016 referendum campaign had dissipated.

Three years on from the referendum, the political dynamics have changed dramatically. Since July 2018 it has been clear that no form of Brexit acceptable to the Tory party can get through parliament. The only Brexit MPs could vote for is unpalatable to the Tory right. Among the right-wing electorate, support for a no-deal Brexit has grown. As defined by who wants it, Brexit is now a right-wing project.

As a result of this, a majority of the electorate now supports Remain. The switch has been strongest among former Labour leavers: working class women, young people and ethnic minorities. As cultural xenophobia has blossomed on the right, the socially liberal majority of the working class have taken to their own cultural barricades.

In these circumstances, Labour’s original tactic — to emphasise the economic issues that unite the working poor across Leave and Remain-voting areas, and avoid a culture war over Brexit — can no longer work. Sticking to it caused the collapse of Labour’s support in the local elections and the European elections. If left unchanged, this dead strategy will for certain deny the party a general election victory.

Corbyn knows this but has wasted weeks refusing to draw the obvious conclusion. Long before the European elections, his advisers were shown detailed polling evidence that the party would suffer mass defections to the Liberal Democrats, the Greens and left nationalist parties: they chose to ignore it, describing worries about the loss of Remain votes as “psychosis”.

They refused, for ten crucial days, to implement a shadow cabinet decision to end Brexit talks with the Tories. Then, when Labour lost half its seats in the European parliament, they sprang into action to brief against Emily Thornberry and Keir Starmer, who had called for a change of strategy.

Now the line is changing. Corbyn is committed to a referendum “on any Brexit deal” (despite rallying people to vote against exactly this at the May National Executive Committee meeting). But the price has been weeks of public disunity.

The entire spectacle looks to the electorate like incompetence, machine politics and the absence of a moral centre. For those of us who supported Corbyn from the first day of his leadership nomination, it is all the more galling because we know from experience that he is a competent, democratic and principled politician.

There were always numerous fractions lined up within Corbynism: the unions needed a party less addicted to privatisation and anti-union laws; welfare recipients were sick of the bullying state; anti-capitalists and environmentalists wanted radical action; plus there was the old, Stalinist wing of the movement, with its perennial obsessions: opposition to Trident, Nato and the EU.

And that’s where the bigger problem begins. All these groups could be contained within a project of anti-austerity and democratisation. Not all of them can be contained within an internationalist response to Brexit. And, as we are now finding out, not all of them are committed to democracy as a means of sorting out differences.

Because the leadership has refused to address this problem, Labour is now polling consistently between 20 and 25 per cent. Corbyn himself has negative personal ratings off the scale of any previous opposition leader.

If it were facing a disunited right, with the Brexit Party determined to split the little England vote, Labour could — even despite these dire numbers — hope to become the largest party after a snap election.

Instead, Farage is pursuing a strategy of electoral alliance with a Boris Johnson-led Tory party which could allow him to take down 30-plus Labour MPs. That would not only hand power to the Tories: it could hand Farage a seat in government. And it would certainly finish Corbyn off as leader.

So the urgent questions for Labour are threefold: can it put together a coalition of progressive voters to rival effectively the coalition of reactionary voters it will face? Can it revive the enthusiasm of a dispirited membership, creating a narrative persuasive enough to win outright power? And can it then — when it meets resistance from the civil servants, fund managers, private intelligence companies and Mike Pompeo’s state department — mobilise a movement on the streets to support a left government in office?

Right now, much of the Labour machine is geared-up to avoiding these questions. So let’s understand why.

For some in the labour movement, the words “working class” have come to mean something wholly anachronistic. When the party chairman, Ian Lavery, barrels around the corridors at Westminster declaring that “the working class wants Brexit”, he means that a section of older, white, former industrial workers in parts of England and Wales want Brexit. And he is right. Judging by what they say in focus groups and how loudly they chant “Ni-gel, Ni-gel” at the Brexit rallies, it is what a section of the working class wants.

Lisa Nandy, the MP for Wigan, argues explicitly that, no matter how far they have drifted away from Labour’s values of tolerance and internationalism, pro-Brexit workers in small towns have a some kind of moral veto over the party’s leftward evolution. She writes that, because they “come from families who worked in the mining, railway and textile industries to build this country’s prosperity and influence in the world”, and have supported Labour for a century, the party has a “historic duty” to them.

I agree. We have a historic duty to tell them that xenophobia and anti-migrant racism are a dead end, just as socialists in London’s East End had to tell this straight to the dockers on the Powellite strikes and marches of 1968.

But our primary historic duty is to represent the actual British workforce of now, which exists in globally connected manufacturing, services and the public sector, is concentrated in cities and large towns, is multi-ethnic, and includes 2.7 million European citizens who don’t get a vote. These are the workers losing their jobs hand over fist at Bridgend, Swindon and Scunthorpe, due to Brexit, and in the case of EU migrants, losing any sense of certainty about the future.

Labour’s path to power is to build an alliance of these workers, together with millions of young voters and, yes, the 41 per cent of the British workforce classed as professional and managerial grades: the nurses, teachers, aerospace engineers, software designers and people in Britain’s huge cultural sector. Call them “middle class” if you want — but they are the backbone of union membership. That is where Labour’s mass membership is concentrated. Its bedrock — as in the people who will mobilise their families come what may — is the black and minority ethnic communities.

There is no sociological basis for the claim that a ex-miner in Wigan is somehow “more working class” than a Pakistani taxi driver in Bradford, or a Greek migrant nurse in Bristol. Or that our historic duty towards the ex-miner outweighs our duty to represent the Lithuanian welder in Dudley. But it is this nostalgic view that underpins the arguments of all Labour’s Brexit supporters, from McCluskey on the left to the Blue Labour brigade on the right.

Howard Beckett, McCluskey’s heir apparent at Unite, claims I want to “trade in the class struggle, abandoning as lost causes millions of working class people, for a culture war instead”. On the contrary, if by culture war, we mean a sustained racist and misogynist onslaught against women and minorities, combined with a concerted push for post-Brexit deregulation, that is the class struggle. And I don’t want to abandon pro-Farage workers: I want to knock on their door at election time and offer them something better.

A trade union, of course, has a responsibility to organise and represent all workers, regardless of political belief — but a radical left-wing party does not. In the present situation, it has to represent people who want to resist austerity, defend democracy, fight fascism and stop climate change. There are very few such people among those chanting “Ni-gel” at rallies organised by hedge fund billionaires.

To put themselves at the head of a coalition of progressive voters, Labour politicians need to speak from the heart, from progressive values, telling a story of resistance and hope. But in the world of Unite, it is these very progressive values that are the problem.

McCluskey told Marr that “a well-funded Remain lobby [has] turned the nation into a toxic situation”. Not Nigel Farage, Arron Banks or Steven Bannon. Not the rancid radio talk show hosts. Not Kate Hoey parroting the DUP or Roger Godsiff parroting anti-LGBT propaganda. But the socially liberal, internationalist young people who swelled the last People’s Vote demo to one million people. By the same logic, Democrats Alexandria Ocasio-Cortez and Ilhan Omar would be held responsible for the rise of white racism in the US.

Its important to recognise that Unite’s position is only a mild version of something much more misguided. Socialist Worker has called the People’s Vote march “an anti-democratic outrage”. John Rees, of Counterfire, claimed that the PV demonstration was a “mild (as yet) variant” of a “mass fascist or populist right wing movement”. Eddie Dempsey, an RMT member of the Full Brexit campaign, said that Tommy Robinson’s supporters were “right to hate the liberal left” for its support for Remain. When the black Labour frontbencher Clive Lewis criticised Dempsey for these words, he was expelled from the RMT’s group in parliament (which still includes both Hoey and Godsiff). In short, a small but influential part of the left is already on the other side of the culture war.

Let’s be honest about the political tradition that underpins all this: it is the shop-soiled Stalinism of the Morning Star and the wider pro-Putin wing of the British left, which now includes many fragments of the Socialist Workers Party. The survivors of orthodox communism in Britain left never advanced a theory of the Soviet Union’s failure; nor did they show much remorse for its crimes against humanity; nor did they acknowledge its record of anti-Semitism; nor did they ever renounce its methodology of front organisations, cliques and bureaucratism.

I have supported Corbyn, and continue to support him, because both as a backbench MP and leader he was always able to rise above bureaucratism, and because his commitment to human rights has been unparalleled during his decades in parliament. And because I knew he could assemble a winning coalition.

The crisis of Corbynism has begun because its economic nationalist wing has claimed the exclusive right to dictate policy and strategy to the rest of us. In a pre-networked era that might have worked — but it won’t now.

It is still possible to reverse the five-party fragmentation of English politics, to save Labour’s seats in Scotland and to recover its primacy in Wales. But not on an unlimited timescale. The voters Labour is losing to the Lib Dems may be reluctant, and may come back; but the voters and activists it is losing to the Greens may find life in a party without a bureaucratic machine so liberating that they never come back.

So the task for Corbyn is clear: to come out fighting against a no-deal Brexit, with a narrative based on values that reconnect his leadership to progressive Britain. That means a struggle in parliament to bring down the Johnson administration, and an election pledge to stop Brexit and rebuild Britain instead. Without this change, Labour’s chances of forming a majority government go from slim to zero, no matter how disastrously Johnson performs.

But even with this change, the fragmented electorate of the left and centre left are going to take some persuading to gather around Labour’s project — and it is Jeremy Corbyn who will have to do the persuading.