Britain’s current political dynamics have been made crystal clear by the Brecon and Radnorshire byelection result. The Liberal Democrats won because Plaid Cymru and the Greens stood down, in an explicit electoral pact. The Tories could have inched it had the Brexit party also stood down. Labour, which never had a chance in this rural constituency, saw its vote share slump from 17% two years ago to just over 5%.

With a majority of just one, it is now inevitable that Britain faces a snap general election – either because Boris Johnson’s government is defeated in the Commons or because, having achieved some shabby Brexit deal, he will seize the moment to look for an electoral mandate.

For progressives, the last two weeks have shown how high the cost of losing a general election would be

For progressives, the last two weeks have shown how high the cost of losing that general election would be. The country would be ruled by a faction of elite Tories who have abandoned their moral and intellectual dividing lines with the far right. Britain would become an appendage of the United States, in foreign policy and in trade. It would be goodbye to the welfare state and the tolerant society.

What to do is the question that’s dominated discussions in the union movement, among NGOs and party activists for the past week. There is only one proven response in history that beats an alliance of far-right populists and conservative amoralists: a temporary alliance of the centre and the left. That’s what the Greens, Plaid and the Lib Dems achieved in Brecon – and it looks like a big chunk of 2017 Labour voters took part in it.

To make it work nationally will not be easy. Everything depends on Labour. Basically its choice is either to lead an informal progressive alliance or to become one. The first thing Labour’s frontbench has to do is to commit – immediately and unequivocally – to fight any general election called before Britain leaves the EU on a remain and transform ticket.

The possibility, still being spun by Jeremy Corbyn’s advisers, that Labour would go into that election equivocal on the issue of Brexit makes my stomach churn. It defies not only principle but also electoral calculation: why bring down a government over Brexit if you won’t tell people what your alternative is?

But committing enthusiastically to remain only gets you to first base. On top of that, we need a one-off electoral arrangement between parties of the left and centre aimed at preventing a no-deal Brexit and removing Johnson from Downing Street.

Facebook Twitter Pinterest Jeremy Corbyn at the Nye Bevan stones at Tredegar, Wales, in 2015. Photograph: Tracey Paddison/REX Shutterstock

I can predict now the screams of protest from many Labour activists. But the popular-front tactic has deep antecedents in the very political traditions the modern Labour left emerged from. In 1935 the Bulgarian Communist leader Georgi Dimitrov single-handedly manouvered the Communist International into supporting calls for a “popular front” against fascism. This was about formal electoral pacts with centrist socialists, left nationalist and liberals – and it paid off within six months. In Spain, to the fury of conservatives who had formed their own electoral alliance with the fascists, the Popular Front took power in January 1936. In May that year the Popular Front won in France, giving the country its first socialist prime minister.

After Neville Chamberlain’s peace deal with Hitler in 1938, Nye Bevan and Stafford Cripps, two key figures on the Labour left, advocated an electoral pact including Communists, Liberals and anti-fascist Tories. Both were expelled from the party in March 1939, but they paved the way for Labour’s wartime coalition with Winston Churchill. So the popular-front tactic is not some piece of niche, retro-leftist memorabilia. It is the property of the western democratic tradition; the only tactic that halted or delayed the march to fascism in the 1930s. And it was invented by the Corbynistas of their day. So how could we make it happen now?

The most obvious part of the proposition concerns the Green party. Up to a million people could simply vote Green as a protest, as they did in 2015, especially since the climate emergency is energising young voters. Labour should offer Caroline Lucas a place in the shadow cabinet now, a place in cabinet if it wins, and to stand down in her constituency and in up to two others where the Greens have a chance. In return, and for one time only, Labour should ask the Greens to stand down everywhere else and to join a united local campaign team. This should be done through formal negotiation beginning now.

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It is impossible to ask Labour activists in Scotland to do the same with the SNP. The progressive working-class in Scotland is too heavily divided between unionism and independence. However, at a UK level Corbyn can offer Nicola Sturgeon something tangible: a penalty-free independence referendum at any point Holyrood chooses to stage it. Brecon was a victory for the Unite to Remain alliance – but I do not propose that Labour joins it. As the official opposition party, it is powerful enough to set its own terms for a limited formal agreement to defeat Johnson. Such an agreement could involve, as Anthony Painter has suggested, a “coupon” agreement, as the Tories and Liberals adopted in 1918. Or the party bureaucracy could simply stop expelling activists who make local-level agreements. Either way, the conversation needs to start immediately – because it will take place with or without the Labour leadership.

With the Lib Dems, Labour has to go beyond simply denouncing Jo Swinson’s voting record. The clue is there in all election results since May: millions of people care more about Brexit than Nick Clegg’s duplicity over tuition fees. I sense cognitive dissonance among many Labour people over the Lib Dem surge: surely it’s not real or permanent? The only way for Labour to win back millions of votes from the Lib Dems is by listening to the reasons why those voters deserted Labour, by proposing a practical unity against no deal, and allowing voters to measure the Lib Dems’ actions against their rhetoric.

Labour activists have to face a grim fact. In addition to being already toxic on the doorsteps of many working-class voters in the north and Midlands of England, antipathy towards Corbyn in many liberal metropolitan constituencies is now high. That antipathy is entirely of his own making. He has looked uncertain on Brexit – calculating and cagey when progressive working-class people expected flair and courage.

But those same voters are, from doorstep reports, also seething about no deal and the money Johnson is spending on preparing for it, while the NHS and local services bleed. To seize back the high ground is a tall order for Corbyn, but it’s possible – with a decisive change of tone and narrative. What he needs to do, and urgently, is show he understands the severity of the threat – to democracy, to the welfare state and to our tolerant society – and to speak from the heart to the party’s twin heartlands: the cosmopolitan cities and the devastated small towns.

The route to power Labour saw in 2017 is no longer open. Even if it claws its way back from 25% to 40%, that will not be enough to form a government if faced with a Nigel Farage-Johnson pact that is backed by dark money and the social-media savvy of the far right. In any marginal constituency where the xenophobic right has one candidate and the progressive left two, the right will win.

Every Labour member should be asking themselves the question: is beating Johnson in a snap election more important than anything else? If you want to beat Johnson, listen to the polls, the professionals and above all the historians – about what it’s going to take to do that. And rule nothing out.

• Paul Mason is a writer and broadcaster on economics and social justice