Brexit-weary as we are, we must gird ourselves for the most significant few weeks for Britain since the Second World War. The crises of our economy, our constitution, our political parties, our identity and even public truthfulness are finally coming to a head. We must save our country from the duplicitous clutches of a zealous nationalist right – and for that the array of opposition parties and independent MPs, with Labour necessarily at its heart, must act.

There is only one way out of the maze. There must be a second referendum, with the facts laid out in the light of the lessons of the past three years. The seismic act of leaving the European Union, with its disastrous consequences, has to be reality-tested in a single-issue confirmatory vote. A general election, with its shopping list manifestos addressing multiple issues, can never provide either the clarity or legitimacy for addressing such a question, especially as it was a referendum that created the mess in the first place.

Project Fear, it turns out, was no project fear. Its forecast of lower investment (now 26% below other recoveries), lower growth (3% below trend as forecast) and a reduction in sterling’s value (18% down) were all correct or erred on the over-optimistic. Investment in the car industry has plummeted from £4.5bn in 2014 to a projected £200m this year. Nothing like this has happened in a major industrial sector since the Great Depression of 1931.

Jo Swinson should not only say yes, she should say publicly that if Corbyn moves she will back him

Under Johnson’s already near-dead Brexit proposal, car companies will wind down their UK operations gradually; under no deal, they will simply close or exit as just-in-time production becomes impossible. Similarly hit: pharma, biotech and aerospace. Even the buoyant service sector is being infected: the IHS Markit business confidence indicator last week fell below 50, foretelling recession in services next year.

Overall, there can only be economic stagnation-cum-recession in 2020 and 2021; if there is no deal, in some sectors it will become a slump. The notion that slow-to-negotiate trade deals with Australia and the US will ride to the rescue is pure fantasy – the volume of trade is a fraction of that with the EU. The EU is an economic safe harbour. More than that. Britain is a European country with a European vocation. There is no deal better than the one we had, being full members of the EU.

Early this week, Jeremy Corbyn needs to make a speech along these lines, setting out the economic realities, and call for an extension of article 50 long enough to hold a second referendum. Labour, he should say, will support a deal with the EU – or an article 50 extension – only if a referendum is held with Remain on the ballot. If Johnson fails to negotiate a deal with the European council on 17 and 18 October, there must be a vote of no confidence and a government of national unity (GNU) formed, which he will lead, whose sole remit will be to take the confirmatory referendum bill through the House of Commons. The GNU will pre-commit to make no new policy, except in emergency circumstances and then only after a pre-declared consultation process. It will be a caretaker, bridge government.

The numbers could be there, if all the opposition forces find common purpose, since Johnson commands only 288 Tory votes and 10 DUP votes.

Facebook Twitter Pinterest Jo Swinson’s support ‘will trigger the momentum to create the interim government’. Photograph: Neil Hall/EPA

The first precondition is that Corbyn must declare that his aim is a second referendum. Already Nicola Sturgeon and even Ken Clarke have said they would support such an interim, single-issue government. The sticking point then becomes Jo Swinson and the Liberal Democrats. So far, the consequence of Swinson’s earlier veto of Corbyn’s offer for a GNU plus a general election, from which she slightly backtracked, has been masked because there is nothing newly viable to veto. But once he has declared for a referendum, she must put any reserves aside and support the leader of the opposition who commands the bulk of the votes that will form the GNU. It is pure realpolitik. The national interest trumps everything and her support will trigger the momentum to create the interim government. She can turn from blocker to enabler.

Yes, I know the anti-Corbyn criticisms – from both social media and in private exchanges with MPs. It is impossible to make common cause with someone who has so connived in antisemitism and been so consistently ambiguous over the EU. Don’t be fooled by his surface decency – he has unleashed the divisive red guard outriders in Momentum on some of the best in the Labour party. And Andrew Fisher, his policy director, when resigning wrote about the “blizzard of lies and excuses” and lack of “human decency” at the top. Corbyn’s radical socialism is so beyond even normal radical leftism that he constitutes a public menace, reflected in personal poll ratings of minus 60.

But... Corbyn is the leader of the opposition. If he is ready to commit to a second referendum along with the votes to make it happen, we must make common cause. Already, millions of lives have been blighted by Brexit; there will be millions more. In life, we do not always choose our political bedfellows – we build coalitions of the willing. This is the transcendent issue of our times. Swinson should not only say yes, she should say publicly that if Corbyn moves she will back him.

Once the referendum is won, which, given the polling numbers and gathering economic difficulties, is Remain’s to lose, the argument in the general election to follow will be how to reset British capitalism and society to address the scale of disaffection that fuelled the Leave vote. Then the Lib Dems can do battle with Corbyn – but from the security of being inside the EU.

• Will Hutton is an Observer columnist