In the atrium of Portcullis House last week, I saw Yvette Cooper and Stella Creasy chatting over a cup of tea. Which got me thinking: it was no stretch at all to imagine these two talented politicians as prime minister and chancellor, discussing the forthcoming comprehensive spending review.

This prompted a further thought: how easy it would be to construct a pretty decent cabinet from Labour’s backbenches: Jess Phillips, Seema Malhotra, Owen Smith, Stephen Kinnock, Gloria De Piero, Angela Eagle and David Lammy for a start. Then add in a handful of existing frontbenchers – Tom Watson, Keir Starmer – and, before you knew it, you’d have the basis of an electable Labour government with a fighting chance of uniting the country and governing it competently.

At which point, Barry Gardiner walked over to greet a colleague – and my private game of Fantasy Government was terminated abruptly by this bleak reminder of what Jeremy Corbyn’s shadow cabinet (with a few honourable exceptions) is actually like.

It was a tiny parable, indicative of a gigantic political problem. In my adult life – and I include 1997 – the nation has never been so obviously screaming for a sensible Labour government.

The Conservative party is exhausted, delirious, badly in need of an Uber off to the quieter pastures of opposition, where it can have a nice little sleep. But it is a huge mistake to imagine that this Uber will arrive of its own accord. The deepest Conservative instinct is to cling on to power, and the only force that can break its grip is Labour.

I now think that Corbyn’s undoing was his undoubted success in the 2017 general election. To achieve 40% of the popular vote was, without question, a remarkable feat. But the conclusions drawn from this performance were not sound. What mattered now, the Corbynites argued, was not “reach-out” – capturing non-traditional Labour voters – but “turn-out”: getting even more of the party’s base to go to the polling stations next time. And so no quarter was given to wicked centrists who were essentially told, in the semiotics of politics, to shove off and move in with Tony Blair if they missed him so much.

“Centrist” is now second only to “fascist” in the left’s lexicon of hate. But there are still quite a lot of voters who fit this description and are looking for a home: appalled by the smallminded nativism of contemporary Conservatism but unpersuaded by the leftwing populism of Corbyn’s Labour. In a YouGov survey last week, his party polled fourth on 18%. This may indeed have been an outlier finding. But come on: no opposition party facing a probable general election in the autumn can possibly be sanguine about the sort of poll results that Labour is achieving right now.

When that election comes, both the Tories and the Liberal Democrats will have new leaders. Corbyn will be old news. What, four years into his leadership, will be the source of his momentum, his drive, his implacable claim to office?

Populism is volatile stuff: it is the political branch of pop culture, and, as such, mostly ephemeral. Who now remembers Corbyn’s star performance at Glastonbury in 2017? Like most hits, it had its moment, blazed across the sky and was then forgotten.

Since then, the Labour leader has failed abjectly to handle the antisemitism row in a manner befitting a future prime minister. The lack of urgency with which he has managed this internal party crisis has been morally contemptible – as is the refrain that every news report on this subject is axiomatically a “smear”.If Corbyn cannot deal speedily with the people in his party’s ranks who hate Jews, what are the odds that he will be able to manage the nation’s education system, or the NHS, or its defence?

On Brexit, we cruise towards a no-deal departure on 31 October. In such desperate circumstances, you do not have to be a remainer to conclude that the public should be consulted again in a fresh referendum. Such an offer, made unambiguously and without qualification by Labour, would electrify the political landscape. Like all such decisions, it would bear an element of risk. But that is the essence of statesmanship.

Whether he likes it or not, this is the great opportunity of Corbyn’s leadership: one he did not seek, but with which he is confronted nonetheless. The goal is two miles wide. The goalkeeper lies unconscious on the pitch. The ball is two inches away from the line. And – thus far – Corbyn’s instinct has been to nudge the ball feebly to the left, promising only to consult his members and the unions on the extent to which goal-scoring is consistent with authentic socialism.

All such arguments are routinely dismissed as Blairite rubbish, the bletherings of the “metropolitan elite”, the voice of those who have “learned nothing”. In response to which I think: oh well, go ahead and lose if you like.

But then I wonder if the clique that has the Labour party in its grip would be the true losers of an election defeat. The people that really suffer when Labour fails are the people it was established to protect: those who need the living wage whose level Boris Johnson does not even know; those who have paid the price for the failures of universal credit; those who need alleviation of the immigration “hostile environment”; a government prepared to tax wealth to subsidise the 21st-century NHS and social care.

At this level of politics, in times like these, feeling righteous is not enough. In fact, in the greater scheme of things, it isn’t anything at all.

• Matthew d’Ancona is a Guardian columnist