Once Brexit is over, what’s left? A two-party system that kills optimism

Brexit breaks everything it touches, scything through the country, risking the union itself. Now it bites chunks out of both main parties, both richly deserving these desertions. More defections may follow: up to 30 from Labour are said to have considered it, while Heidi Allen claims a third of Tories would jump if no deal is in prospect – Justine Greening and Dominic Grieve have confirmed they are among them.

This is as it should be, because Brexit is the most momentous political crisis of our lifetimes, overriding all other political issues, crossing party lines. Right now, in these crucial weeks, every good MP should put the fate of the country first and do all they can to stop Britain teetering over the Brexit precipice. They should threaten their rigid leaders with whatever damage they can inflict to make them change direction.

Theresa May should long ago have ruled out any possibility of no deal. By (failing) to appease her Brextremists, she has already caused the flight of bankers’ billions – £800bn gone already according to accountants EY, more than the Exchequer’s entire annual public expenditure. By letting the no-deal threat run, it has caused companies to flee and business has disinvested permanently. It has cost the Treasury £4.2bn for the government’s own pathetic no-deal preparations alone.

Jeremy Corbyn likewise deserves the damage done to his party’s electoral support for refusing to lift a finger to stop Brexit, and failing to back the referendum that is his party’s policy. Labour is not in power and not to blame, so anything he does to enable Brexit will break him and his party from the roots upwards. How easy it should be to oppose this far-right Tory endeavour, decades in the brewing, with the most distinguished Tory grandees – John Major, Kenneth Clarke and Michael Heseltine – on the opposition’s side. It takes a genius of obstinacy and incompetence to fail to lead the national opposition to the coming cataclysm. Open goal, own goal? No, this isn’t sport.

Maverick Brexit enablers in his ranks, the Caroline Flints or the softer Lisa Nandy tendency – are insignificant compared with the overwhelming remain battalions of Labour members and voters. Even in leave seats, the majority of Labour voters are remainers – and leavers have left. Peter Kyle and Phil Wilson have devised an easy path for Labour to heal its own divisions: their motion would nod through whatever deal May brings home, in exchange for her putting it to the public to decide. But that requires Corbyn and his anti-EU inner coterie to mount a mighty Labour remain campaign – and mean it. Could they, would they? If not, the party will fall apart.

How Brexit plays out will determine the future shape of British politics: a crash-out will finish the Tories for ever. The problem now is that defections don’t change the parliamentary stalemate, with no majority for anything. The prime minister still needs 60 of her MPs to switch and back her deal, while remainers need to woo enough MPs to back the Kyle motion.

But let’s be optimistic and imagine these rebels help bring off the best possible ending: article 50 is delayed, there is a great national debate with public citizens’ assemblies, and finally a referendum: remain wins with a bigger majority than leave won in 2016 – though leaving political scars.

What then of the new political entity created this week? Once you remove Brexit, what’s left? In these three breakaway women MPs, the Tory party has lost some of their best, most human faces who could yet be joined by others of their ilk – say, Nick Boles, Jo Johnson and Sam Gyimah. But it is not tribal to point out that nice Tories are still Tories, as Anna Soubry and Sarah Wollaston eagerly confirmed that in their departure. They both still think extreme austerity was necessary, and they backed it – yes, even the nice GP who saw its effect on her NHS.

They all give the oddest reason for deserting, that May has ditched a Tory “modernising” project – but it was all Cameron style and no content. David Cameron intervened to claim, outrageously: “I respect their decision, but disagree with them: we need strong voices at every level of the party calling for the modern, compassionate Conservatism that saw the Conservative party return to office.”

What compassion was that? He and George Osborne inflicted harsher cuts than Margaret Thatcher ever dared, still searing through all public services and benefits, bankrupting local government, leaving a million sick old people with no care, schools stripped of funds and an economy impoverished by austerian fiscal policy. Brexit has not killed left and right, the eternal divide.

Can a new party blend Labour and Tory MPs and attract more than a smattering of the Labour voters it needs? Anyone can change their mind, anyone can be forgiven past errors, but these Tories haven’t repented, Soubry still praises her old party’s “sound finance”. What would be their economic policy? Alliance with equally nice Vince Cable and his Lib Dems would be similarly fraught: he and Nick Clegg have repented none of their part in the coalition’s utterly destructive policies. Only the Greens look a safe partner – but surely they wouldn’t bind with Tories?

Of all Clegg’s coalition sins, the worst was his failure to secure the one reward worth having – proportional representation. Without it, political startups are doomed. There should be space for a socialist and a social democratic party, for moderate and far-right Tories and Ukip types. The Greens would ride high. Without PR, voters are forced to hold their noses and vote against their most hated of the two old blocs. That breeds political cynicism and kills enthusiasm. The Independent Group, bouncing with optimism, may break all the old certainties: more likely the old certainties of first past the post will break them.

• Polly Toynbee is a Guardian columnist