Being on the Labour left at the moment feels a lot like how I imagine the last days of Rome. For a brief moment, we conquered the Labour party; now we are contemplating what to do with the some of the worst poll ratings in its history. In Jeremy Corbyn’s office, shadow Cabinet members stalk the corridors, pouring wine into the open mouths of passing press aides and advisers, many of whom are rushing to the exit. Most Labour MPs, having spent two years tearing down the city’s defences, cheekily shrug their shoulders. “We tried to stab him – hundreds of times”, they say. “Now let’s just wait for the end."

If the world feels like it is collapsing, that is because, on many fronts, it is. The Copeland by-election was the first time an opposition party had lost a seat in a by-election to the government since 1982. Even in London, Corbyn is, according to one poll, less popular than Paul Nuttall. Brexit is forcing Labour into such contortions that rising star Clive Lewis felt compelled to resign from the front bench. Across Europe, the new radical left is stagnating, although doing better than most social democrats, who are being annihilated. Meanwhile, the orange shadow of Donald Trump, the looming breakup of the UK and the faint prospect of war over Gibraltar give the situation a certain surreal, cinematic quality.

Demoralisation is a natural feeling for much of the left’s base. The new leadership promised to change politics, to give political expression to social movements, and to bring Labour the intellectual clarity it needed to cut through in an era of polarisation. That entire project is now on the brink – and it isn’t because of a few bad press performances. It is because, unlike much of its base, the leaders of the British left do not understand what Brexit means or how to deal with it.

The idea that Corbyn secretly backed Leave is a myth, but it is certainly true that Labour is now failing to grasp the scale of what is happening. Brexit isn’t just part of a global right-wing populist insurgency, mirrored by Trump across the Atlantic. What the publication of the Great Repeal Bill has demonstrated is that it is also a minefield of legal and legislative measures – and with a whole new window for corporate influence on the state – that will rewrite Britain’s constitution and fatally undermine what little democracy we have.

Yet as Corbyn rose to the despatch box last week to challenge May on her Brexit strategy, having spent PMQs talking about police cuts and school funding, he did so having voted for it at every possible opportunity, and reserved his strongest objections for the government’s “complacency”. Faced with the worst political and economic crisis in living memory, Labour seems to want to talk about other, easier things. To move forward, it must bite the bullet and take sides.

Taking sides in the Brexit debate isn’t necessarily about trying to remain in the EU. It certainly isn’t about becoming the party of the 48 per cent; that strategy may work for the SNP, but would ruin Labour. It is about drawing up a series of clear policies and fighting for them –and that ought to be the Labour leadership’s comfort zone. Brexit and Article 50 may have divided every area of the Labour movement, but the real battles – on free movement, for example – should be left-right issues. The right should be expected to pedal the idea that immigration is to blame for the housing crisis and falling living standards; the left should put across clearly articulated class politics as the alternative.

On the processes of Brexit, and on the issues that surround it, Labour suffers from a cocktail of toxic influences. On the right wing of the party, MPs openly advocate tougher immigration controls. On the left, two key intellectual influences are the Bennites and a school of thought traceable to the old Communist Party – who have in common a Eurosceptic, nationally rooted conception of the Labour left project, in many ways a reheated version of post-war social democracy. Add to that the chronic tactical conservatism of Labour as a whole and you get a party which cannot connect to either reality or its supporters on the main issue of the day.

With bold enough ideas, this could be a moment of renewal and opportunity for Labour – hitting back against nationalism, selling a forward-looking transformative politics to a mass audience. With basic assumptions about the direction of history – social progress, civil rights, limits on executive power – being eroded, there are all kinds of new allies and audiences for a form of socialism that can articulate a sharp alternative to the populist right and connect with a new layer of politicised people.

Like it or not, radicalism is still Labour’s only hope of electoral success, and the Corbyn project still carries the hopes of the left across much of the world. To save it from a spiral of demoralisation, Brexit must become a fight to be relished, not an awkward side issue.