Who will speak for liberal Britain? Not the SNP, for a start, which understandably wishes only to speak for Scotland, even as it provides more coherent and determined opposition to the Tories than Jeremy Corbyn’s demoralised party.

Explaining in a speech at Bute House, Edinburgh, on 13 March why the SNP wanted to hold a second independence referendum, Nicola Sturgeon stated as a self-evident fact that the Labour Party had “collapsed” and that the Conservatives would be in power until at least 2030. Such sentiments have hardened into received wisdom and pass unchallenged by Labour MPs.

Delivering his first Budget as Chancellor, Philip Hammond taunted his opponents by referring to the last Labour government as being the last Labour government. Speak to any Conservative MP and he or she will say the same thing: they feel no pressure from the Labour opposition. More, they will say that there is no opposition. Period.

The Labour leader’s left-wing media cheerleaders have, one by one, given up on him. Charlotte Church, Caitlin ­Moran, Owen Jones, George Monbiot, Zoe Williams: all invested considerable hope in Corbyn, who has not turned out to be the inspirational leader for whom they yearned. Even Simon Fletcher, who masterminded Corbyn’s leadership campaign in the heady summer of 2015, has quietly walked away.

From the beginning we were opposed to the Corbyn leadership but, in the spirit of plural debate, happy to open our pages to him and his confidants. Our view was that Corbyn was ill-equipped to be leader of the opposition and, indeed, an aspirant prime minister. Irrespective of his ideological obsessions, there was nothing in his record as a parliamentarian to suggest that this serial rebel would have the organisational ­capacity to unite his party and evolve a far-reaching, transformational policy programme. There was nothing in his record to suggest that he could remake social democracy or under­stand, let alone take advantage of, the post-liberal turn in our politics. The decline of Labour pre-dated Corbyn’s leadership, of course, but he and his closest allies have accelerated its collapse into irrelevance.

We accept that, after the traumatic defeat of Ed Miliband and Labour in 2015, activists were despondent. Corbyn was an unapologetic socialist, unembarrassed by his long career of rebellion from the back benches. He was a passionate anti-capitalist. His determination and consistency appealed to those who value stubborn principle over pragmatism and who loathed Tony Blair, or at least what he became. Students who knew nothing of the Bennite wars had never before heard a front-line British politician speaking as Corbyn did at “anti-austerity” rallies during that late-summer reawakening of radical socialism in 2015. And as the rebel insurgent he was untainted by the inevitable compromises of power.

Corbyn evidently unlocked something long repressed on the left. David Cameron’s England was characterised by public penury and private ostentation. Labour activists were sickened. They wanted an alternative and believed Corbyn would provide it.

Why, even his dishevelled appearance and clipped beard gave him a certain boho, hipster chic. Unlike the tortured Ed Miliband, Corbyn knew his own mind. He knew what he wanted to say and how to say it – because he’d been saying it ever since he entered the Commons in 1983. Corbynism was meant to be a counter-hegemonic project. It was meant to herald a “new kind of politics”: gentler, kinder, dynamic, more progressive. But what is most striking about Corbynism – apart from the dysfunctionality and incompetence of the leader’s office – is its intellectual mediocrity, its absence of ideas.

In the 1970s, as those who would later be called Thatcherites set about dismantling the postwar consensus and creating a new economic settlement, the sense of intellectual ferment was thrilling. There is no comparable sense of intellectual excitement on the Corbynite left. It’s as if Corbyn has nothing of substance to say.

“A house divided against itself cannot stand,” said Abraham Lincoln. Labour is fatally divided inside parliament and outside it. On its present foundations this Labour house cannot stand. The MPs do not want the leadership. The leadership does not want the MPs; it wants to unhouse them. Corbyn, with his self-deprecating humour and Orwellian eccentricities, is a considerate man – as I discovered when I spent a day with him in Prague just before Christmas – but he is not a leader, even more pressingly so at a time of national emergency. Corbyn has failed even on his own terms, and his failure has created a crisis of the left but also, more optimistically, an opportunity for some kind of realignment.

In the film adaptation of James Ellroy’s LA Confidential, Officer White (Russell Crowe) makes a home visit to an elderly woman whose daughter is missing. There’s an unpleasant smell coming from the basement. “A rat died behind a wall,” the woman, who is called Mrs Lefferts, says. Crowe investigates and discovers a decomposed body hidden under some sacks. “Was it a rat?” Mrs Lefferts asks. “A big one,” Crowe says. It’s as if the woman had grown used to the smell and could tolerate it as one tolerates changes in the weather.

It is something like this now with Jeremy Corbyn’s Labour. The electorate can smell that something is seriously wrong and is recoiling, but those closest to the triumvirate of the leader, John McDonnell and Diane Abbott seem oblivious to or unconcerned by the stench of failure. Meanwhile, as a consequence of Brexit, the fractures in the Union widen and deepen, yet Labour abandons all pretence at competent and unified opposition. And so the question remains: who will speak for liberal Britain?