Why now? That’s the obvious question arising from yet another bruising encounter between Jeremy Corbyn and his frustrated MPs, widely described as the worst meeting since the last one.

The charge sheet is certainly serious, ranging from a failure to lead over Brexit to the way sexual harassment allegations against his aide David Prescott were seemingly brushed under the carpet and a long-standing reluctance to act on allegations of antisemitism (this time levelled against the new Peterborough MP, Lisa Forbes). But these are not fundamentally new problems. Labour MPs have been round this track again and again, and got nowhere. What is new, however, is that the whiff of a general election in the air has significantly raised the stakes.

Labour’s fitness to oppose, and then govern, coherently in the national interest has never mattered more

Anything could happen to the frontrunner in a Tory leadership race, but there is now a very real prospect of Boris Johnson winning it. The odds of a no-deal Brexit are shortening, and his promise of a billion-pound tax cut for the rich, just as the economy shows signs of shrinking, suggests an alarming willingness to tell any given audience (in this case Tory members) what they want to hear, regardless of the consequences. Johnson must know that, as Andrea Leadsom pointed out, a hung parliament could easily reject such a tax cut just as it will try to reject no deal. But so far it looks suspiciously as if he’s campaigning for the leadership much like he campaigned to leave, by promising things no responsible opponent can match on the assumption that he might not even have to deliver. His rivals argue all this would be so divisive that it makes an early general election, or possibly even a second referendum, more likely and they are not necessarily wrong.

So far, so good for the Labour party, especially after the Peterborough byelection suggested its Brexit position hasn’t split the leftwing vote or demoralised its activists quite as badly as some thought. But it also means Labour’s fitness to oppose, and then govern, coherently in the national interest has never mattered more. For some, the prospect of going back on the doorstep raises long-standing anxieties about how they could in good conscience campaign for Corbyn to become prime minister.

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The understandable fear among Labour remainers is that the leadership will see its narrow escape in Peterborough as vindication for its pro-Brexit stance. If nothing changes, they may shortly be expected to look their constituents in the eye and argue in defence of a position with which they are deeply uncomfortable and which they genuinely feel will make those constituents poorer. Others feel exactly the same about the prospect of asking Jewish constituents to vote Labour when the party couldn’t even bring itself to suspend and formally investigate Forbes, who not only liked an antisemitic post on Facebook – an honest mistake, she says – but also commented under a rambling post about how the CIA and Mossad supposedly created Islamic State. She wrote: “I have enjoyed reading this thread so much.”

They are right to argue that voters deserve better than being asked to condone one form of racism in hopes of avoiding another, or pushed into choosing the least worst of Brexit evils. And with a new Tory leader hovering in the wings, time is running out for Labour to clean up its act. If not now, then when?

• Gaby Hinsliff is a Guardian columnist