The Conservatives are plotting to overturn Britain’s constitutional order to stop them haemorrhaging votes to the Brexit party in key marginal seats. The most absurd aspect about this sentence is that it’s entirely true.

The country’s current turmoil has two proximate causes: the Tories’ decision to make the majority pay for a crisis caused by their donors – the financial sector – breeding mass disillusionment and anger; and the Tory addiction to making calamitous decisions driven by their own perceived partisan interests. That’s why David Cameron called the Brexit referendum; why Theresa May bungled the negotiations, polarised the nation and mainstreamed the idea of no deal; and it is why Boris Johnson intends to engineer a disorderly Brexit during an election campaign.

A warning for Johnson: May and Cameron played with matches, and they self-immolated.

Rather than being a single-issue party, Labour will also offer radical pro-environment and anti-austerity policies

This is a full-throttled assault on a democracy that was won through struggle, sacrifice and much spilled blood. No deal was never a proposition during the referendum campaign; the electorate were left with the impression that a negotiated agreement would be swift, easy and painless – that we would “hold all the cards”, as Michael Gove put it. Polling consistently shows that a no-deal exit from the EU is a position endorsed by only a minority of the electorate. If the Tories want a democratic mandate for this act of self-kneecapping – which will leave us recommencing negotiations with the EU from an even more enfeebled state – then they should offer that as a choice in an election, rather than as a fait accompli, whoever wins a subsequent vote. You can see their cynical gameplaying: they think they can unite leave supporters while remain backers are divided, that Labour will be sent into an internal tailspin about offering a soft Brexit or rejoining the EU, and that there will be no immediate pain felt by crashing out of the EU without a deal to exact a political cost on the Tories – just a tidal wave of leave triumphalism for Conservatives to surf.

That’s why Labour’s letter asking Sir Mark Sedwill – the cabinet secretary and Britain’s most senior civil servant – to intervene to prevent a disorderly Brexit mid-election is canny and important. It is reported that there is a split between the attorney general, Geoffrey Cox, who argues the Tories can legally orchestrate this undemocratic absurdity, and Sedwill, who does not. Purdah forbids major government policy decisions during the run-up to an election, after all, and most constitutional experts seem to agree with Sedwill. But Sedwill reportedly has his eyes on the position of British ambassador to the US, and therefore may be fearful of incurring Johnson’s wrath. If Sedwill throws in his lot with the Tory no-deal extremists, and seeks to throw his country off a cliff edge for the sake of his own career, he will secure his name as a villain in history books for all time.

What of Labour’s next move? There is much chatter of an emergency national unity government for the sole purpose of blocking no deal and calling an election. Labour is already being blamed for the Tories’ planned act of extremism by the likes of Chuka Umunna – a member of the Lib Dems at the time of writing – unless it submits to backing such an administration, headed by a backbench MP most of the public would never have heard of. This is an absurdity: the leader of the opposition – a twice-elected leader whose party secured 40% of the vote in a general election just two years ago – has the biggest democratic mandate. If the Lib Dems refuse to make him a temporary pre-election prime minister – justified by reheated Tory lines about Corbyn’s threat to national and economic security – then why aren’t they to blame for what comes next? In any case, a national unity government is an unnecessary distraction: as the journalist Stephen Bush puts it: “If a majority in parliament can be found for a government of national unity, then a majority will certainly already exist for the legislative action to stop no deal by other means.” This must now be the focus.

But Labour has challenges to face, too. Its initial reaction to Johnson’s assumption of power was weak, and its Brexit position needs more clarity. Johnson’s adviser Dominic Cummings – now co-prime minister – is betting on the Lib Dems and Greens dividing the anti-Tory vote to secure a majority for the most extreme British government of our time. The Greens are buoyed by their European parliamentary success and their polling – but have already haemorrhaged many left-leaning members to Labour: whether they stand down in marginal seats, as they did last time, is uncertain. While voters must be made aware of the consequences of voting for other parties – letting the Tory extremists win through the middle – Labour must focus on a positive case: it will put forward a referendum with remain on the ballot paper but, rather than being a single-issue party, will also offer radical pro-environment, anti-austerity, pro-public ownership, anti-tuition fees policies.

We are on the brink of national disaster. There’s still time left on the clock to avert it – but very little indeed.

• Owen Jones is a Guardian columnist