Boris Johnson could be prime minister in a month. Is Labour ready to take him on?

The nation watches this strange array of men deranged enough to aspire to run a country in an insoluble crisis of their own making. As a despairing and departing chancellor spells out today at Mansion House, they all lie through their teeth: there will be no new EU deal, they will be stopped from leaving with no deal, they will not keep their wild tax-cutting promises – and they know it.

Yesterday they ejected the only one who tiptoed closer to these truths, but Rory Stewart also swore blind he would Brexit, come what may.

We watch aghast at the grisly choice, outraged again at being excluded from electing our own prime minister. Our fate is left to a rabble of extreme rightwing revolutionary Tory members who will tear up everything in pursuit of an abstract ideological phantasm. They may look like deceptively normal Rotarians and golf-club members, pillars of their communities, but YouGov finds them eager to wreck the union: 63% glad to see Scotland go, 59% waving goodbye to Northern Ireland in order to see Brexit take place. But most astoundingly, nearly two-thirds of this erstwhile “natural party of government” are quite sanguine if their Brexit does “significant damage to the UK economy”.

Jeremy Corbyn and John McDonnell feature strongly in these Tory debates as mortal hazards facing the nation – looming Marxists ready to turn us into Venezuela. But in this world turned upside down, nothing ever proposed, planned, desired or dreamed by Labour’s left, nothing in its natonalising manifesto, ever matched these Tory Maoists-on-speed’s willingness to wreak “significant damage” on us all.

There never was a more open goal for Labour, as day after day the effects of a decade of malevolent maladministration tumble out. On Wednesday, on just one ordinary news day, the Organisation for Economic Cooperation and Development (OECD) reported UK teachers have the heaviest workloads, bar Japan. The head of NHS England found the 10% cut in hospital beds has been too many (who knew?). The number of rough sleepers in London reached a record high. The Resolution Foundation found 18- to 29-year-olds’ spending money has fallen 7% since 2001 while the over-65s’ has risen 37% over the same period.

Labour should be stratospherically ahead in the polls. Instead, its 14% showing in the EU elections, behind the Lib Dems and only just beating the Greens, shows Brexit blows away everything else, trumping old party loyalties. Pointing to austerity’s devastations doesn’t summon back those lost Labour votes.

Fence-sitting has been calamitous: YouGov shows Labour has lost half its 2017 general election votes – a third to remain parties and 11% to Brexit parties. As the YouGov founder Peter Kellner points out, an average of the last year’s polls shows a national swing – with 44% backing leave and 56% remain. This is partly demographic (older, mainly leave, voters are dying while pro-remain teenagers reach voting age) but crucially the swing to remain is bolstered by Labour voters in the north and Midlands changing their minds.

The shadow cabinet meeting on Wednesday responded slower than a slug, while Labour members agitate frantically for a rapid response U-turn. A report presented, purporting to show backing remain would lose more seats than fence-straddling, has been forensically dismantled by Kellner and by the psephologist Lewis Baston. In virtually all leave constituencies, most Labour supporters voted remain: non-Labour supporters created leave majorities.

Those at the meeting say the report cut no ice – and there was marked shift in attitudes: only Ian Lavery, the party chair who ignores party members, and Jon Trickett stayed adamant Brexiters with a new softening of opposition from the likes of Richard Burgon and Andrew Gwynne. The rest moved with Corbyn’s confirmed position, to support a referendum. But that’s only halfway. At next Tuesday’s meeting, they will have to move to backing remain. Nothing else will do. Said one, “How do you back a referendum but when asked what Labour supports, reply ‘We don’t know’. Ridiculous!”

Despite references to Harold Wilson in 1975 letting his divided party take opposite sides, that can’t and won’t happen this time. (Though Brexiters may be recused in a campaign.) The party is not deeply divided, but very strongly remain in parliament and constituencies. Powerful voices – Keir Starmer, Tom Watson, Emily Thornberry, Diane Abbott, Andy McDonald and more – show why remain is the only credible Labour policy. Despite 26 of Labour’s MPs writing a letter pleading against a referendum and remain, here’s the question for them: will they really vote for any Boris Johnson deal (possibly a joint Nigel Farage/Johnson deal), and trust this most treacherous man with the crucial next stage of EU negotiations? To back it will be to sabotage the Northern Ireland border and the Good Friday agreement. Surely, come that day, they won’t, they can’t.

Optimists in the shadow cabinet hope that as soon as next Tuesday Jeremy Corbyn will shift to remain. But will he sell it, argue for it, make the pro-European case, or skirt around it again, leaving it to half a mumbled phrase tucked into speeches on austerity? Rachael Maskell, a strong Corbyn-supporting MP, but co-founder of Love Socialism Hate Brexit, talks to him often. She has urged Corbyn “to lead the charge. He’s a visionary, but he’s being choked off by the push-and-pull and by his inner office. We should be leading a big, bold reform in Europe, on climate, on globalisation, not a little England at the mercy of a Trump trade deal.”

Maskell wants those in leave seats to campaign as she did nonstop in her poorest working-class estates, successfully swinging them with proof that Brexit would be terrible for them.

The time is short. Johnson will likely be prime minister in a month, and by then Labour needs to be his most powerful, coherent, unequivocal opposition – or others will. Labour’s flannelling half-assent to Brexit gives daily aid to enemies of everything Labour stands for. Every day that passes, the less forgiven Labour will ever be by those who have already deserted it and may never come back.