At a rally on the eve of Labour’s annual conference in Brighton, a party supporter called out from the crowd to ask the leader when he was planning to bring back socialism. “That is what we are doing, sir,” came the reply – and it was considered daring enough to be news.

That was 2013; the leader was Ed Miliband. Later that week, he unveiled plans to freeze energy bills, which Conservatives took as proof that the man was an anti-capitalist maniac. (A diluted version of the policy found its way into the 2017 Tory manifesto.)

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Four years later, also in Brighton, Labour has moved on from bill-capping to full-scale renationalisation. And not just of energy companies. Railways, Royal Mail, the waterworks: “We’re taking them back!” John McDonnell told the conference to delirious applause on Monday. It will not be news if Jeremy Corbyn announces in his speech today that he is a socialist.

Corbyn and McDonnell are proud to have redrawn the left boundary of what an opposition party in Britain can pledge and still be considered a plausible prospect for government. The Tories keep saying that such an outcome would bring catastrophe, but since the election they cannot be sure that voters will heed the warning. Theresa May’s hopes of a whopping mandate were built on the idea that Corbyn was a ridiculous, sinister candidate for prime minister, and she lost her majority. Conservative MPs fear a rerun would tip them into opposition.

It didn’t help that the Tories have been crying socialist wolf for years. It wasn’t just Miliband who was cast as a fanatic. When Gordon Brown authorised the emergency nationalisation of Northern Rock, George Osborne’s instant reaction as shadow chancellor was to denounce the move as a regression to 1970s-style state meddling.

Ten years on, Corbyn’s antagonists vastly overestimate Britain’s cultural abhorrence of radical leftism as a potential conveyor towards economic ruin, possibly tyranny. The cold war is ancient history to first-time voters. To remember two Germanys you have to be well into your 30s, and the memory alone is not enough to guarantee suspicion of grey-haired politicians who once equivocated over preference for the western one. It is not enough even when those politicians, guided by some doctrinaire muscle memory, still tilt towards the Kremlin on foreign policy.

But the big change is that, with Brexit, the Tories have normalised all forms of radicalism. By hurling themselves at a plan in defiance of sober economic counsel, Conservatives have raised the bar for what counts as a dangerous gamble with the nation’s prosperity; they may have removed it altogether. They march to the drumbeat of revolutionary fantasy, urging sceptics to believe that anything is possible with an effort of will. The prime minister touts a panacea policy that is meant to satisfy every conceivable social and economic grievance. She will spare no expense to get it done, leave no magic money tree unshaken. When that is the ethos of government, ministers can hardly complain if the opposition takes the same approach.

Having listened to the debates in Brighton, it strikes me that the unworldly utopianism inherent in Brexit attracts Corbynites, regardless of whether they voted for it. Labour is packed with remainers, but there is marked reluctance to pine for the EU, reflecting the calculated ambiguity of the leadership. This is not simply a question of respecting the referendum result (Labour delegates, after all, are in no hurry to recognise May’s authority as a fairly elected prime minister). Nor is it purely tactical. Yes, there is an electoral conundrum posed by the volume of leave voters in Labour seats. But the pre-referendum spirit of Corbynism celebrated the possibility of changing minds and reframing debates, not trimming ideological sails to any wind. That was shabby Blairism, once upon a time.

Brexit has a deeper lure, conveyed by the Labour leader’s suggestion in a pre-conference interview that membership of the single market could impede the pursuit of a radical left manifesto. This belief rests on aversion to European rules regulating state subsidy for industry and enforcing fair market competition. That provokes a question of quite how radical Labour intends to be. EU membership poses no insurmountable obstacle to nationalised utilities, redistributive taxation, generous welfare states or activist industrial strategy, as a glance at France, Denmark or Germany shows. Even Labour’s new friends in Greece’s Syriza party – no cheerleaders for neoliberalism or austerity – recommend sticking with the European project. If Corbyn could not be satisfied with egalitarian social democratic policies practised for decades in Scandinavia, if he is shopping for a system beyond the leftmost frontier of EU rules, he must be looking at something altogether more drastic. Cuba, perhaps.

Labour has all the permission it needs to be radically unbound, because Brexiters have incinerated the rulebook

Labour’s new radicals respond that comparison with existing forms of government is narrow-minded, pessimistic, tethered to the wreckage of 20th-century thinking. The goal, they say, cannot be grasped without a liberating bound of the imagination. Here too they sound like Tory Brexiters, who promise that the UK will carve out a unique role in the world, starting with a bespoke EU partnership bringing all the benefits of membership and none of the constraints. When that ambition is tested by mundane realities of trade and diplomacy, the utopians insist that reality yield. Corbynism sits awkwardly with anything that smacks of remain because it is intellectually allergic to incremental change, and romantically impatient with everything about the status quo.

It is an extraordinary feature of British politics that two years ago there was no revolutionary party in the mainstream, and today revolution is the only item on the menu. Tory Brexiters and Corbynites face in opposite directions but are held together by mutually reinforcing dynamics. Fear of a Labour government keeps moderate Conservatives from breaking ranks and disrupting May’s plans, lest they end up triggering an election. And Labour has all the permission it needs to be radically unbound, because Brexiters have incinerated the rulebook on economic prudence and moderation. May has deployed the full capacity of government to make revolution respectable, which gives Corbyn and McDonnell their chance to make revolution permanent.

• Rafael Behr is a Guardian columnist