The great Brexit crisis slices through both parties, dividing families, friends, neighbours and colleagues. It may yet break apart the moribund political system. But that seismic rupture didn’t happen today when seven MPs walked out of the Labour party.

Seven is a pitifully small number. The timing is monstrously badly judged and the reasons the MPs give are oddly scattergun, lacking political punch and focus. To be sure, they are not alone in thinking Jeremy Corbyn a weak leader with many failings: his poll ratings show most of the country agrees, as did the 172 Labour MPs who voted no confidence in him two and half years ago, as his “kinder gentler politics” turned poisonous.

But whether born of despair or vanity, this walkout is a damaging distraction, because right here, right now, there is only one cause that matters – Brexit. That’s not one issue among many, it is the great question that has the nation’s future hanging by a thread. It is the debate that contains within it all the other arguments about Britain’s ideals, identities, ideologies and insanities.

If Corbyn makes Labour complicit in this historic Tory catastrophe, then expect all hell to break loose in the party

Those other MPs who tried to oust Corbyn still think much the same of him now as they did then. But they have not quit, not at this catastrophically inappropriate juncture, diverting attention from the supreme task of this generation of politicians. The best of them, such as Peter Kyle and Yvette Cooper, are stuck deep into battle to rescue us from calamity: Labour will again back Cooper’s amendment next week to prevent a no-deal crash-out and delay withdrawal. Some of those 35 Labour MPs who failed to back it last time are being brought round, giving it a good chance of success.

It does of course madden many Labour MPs that foot-dragging Corbyn has been almost absent from the fight against this Tory Brexit disaster. With him surrounded, if not held captive, by a cabal of Len McCluskey’s people, you only had to listen to the Unite leader’s infuriating pro-Brexit views on the Peston show last week to suspect he spoke the Labour leader’s mind too. Their views are deep-frozen in the 1970s. But what matters is less what Corbyn privately thinks than what he does when it comes to crunch Brexit votes – and so far, there has been no misstep.

Labour MPs walking away at this point only give succour to those Labour pro-Brexiters,such as Caroline Flint, who caricature those in favour of reversing Brexit as members of “metropolitan elites”. Paradoxically, their defection hardened the resolve of scores of Labour MPs to stay, those who in the past considered splitting. They are the ones most angry at the defectors, accusing them of failing to work hard enough in their constituencies, failing to build around themselves loyal teams to fight off marauders. The one who has their sympathy is Luciana Berger, bombarded with antisemitic abuse and forced to fight off deselection when about to give birth.

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This walkout comes as a head of steam is building up within Labour at Corbyn appearing to dodge his party’s conference policy to back a referendum. Corbyn’s own Momentum movement has been expressing its frustration at their leader’s reluctance to get behind a people’s vote. Labour has been haemorrhaging members, old and new, over Brexit. Wherever I go, Labour people tell me they are struggling to persuade members not to tear up their cards. “Don’t do it!” they say. “Stay and fight for Labour to take the right stand on Brexit. Corbyn won’t be there much longer: stay to select the next leader, don’t abandon us.”

Does this feel at all like 1981 and the creation of the Social Democratic Party (SDP), which broke away from Michael Foot’s Labour? Not a jot. I was there and there is no comparison. These seven lack anyone of the stature, public recognition or intellectual heft of the SDP leaders. The Gang of Four between them had all held serious government office – between them they had occupied the roles of chancellor, home secretary, foreign secretary, education and transport secretary. This grouplet does not have that kind of weight and it lacks ideological substance and ideas. Corbyn’s failure to stamp out antisemitism is a just cause, but why not stay to fight it?

Remember, the SDP defectors left not out of irritation with an inadequate leader, but because they could not and would not stand on – among other things – Labour conference pledges that a future government would abandon nuclear weapons and take Britain out of the European Economic Community. (Labour went on to entrench these pledges in the manifesto under which Foot fought the 1983 election.)

Compare that to Corbyn and John McDonnell’s last manifesto, which was widely popular, with no ideological deal-breakers for a broad church party. Some might query costings or priorities – why billions for middle-class students, not for needy toddlers in Sure Starts? Some might niggle at the practicalities of utility renationalisation – but these were popular policies. Nothing there felt like an updated version of the notorious 1983 manifesto, dubbed the “longest suicide note in history”. These seven defectors were vague on reasons for jumping ship, beyond wishing for a better leader.

Philip Collins , a former speechwriter for Tony Blair, has written a good manifesto for a putative new social democratic party, entitled Start Again. But since almost any Labour MP would back almost all of it, that suggests less need for a new party than for one better led. Brexit could still destroy Labour. The 2017 manifesto got away with promising “the benefits of the single market and the customs union”. Since then, Keir Starmer has deftly led the party to vote the right way on every Brexit division. So far so good.

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But now Corbyn needs to back the Peter Kyle/Phil Wilson clever compromise. Their plan is for MPs to agree to pass Theresa May’s bad deal, but only on condition it is put to the voters for a final decision. People on both sides of the divide are gathering round this option as the best chance of resolving Brexit, once and for all, whenever May finally holds her meaningful vote. The plan lets MPs in Brexit-voting seats obey their electorate by voting for May’s Brexit – but frees them to campaign to remain in a referendum. The good signs are that McDonnell and Starmer are warming to the plan. This will be Corbyn’s vital test: in the end he will have no choice but to do the right thing.

If the Labour leader ends up in any way enabling Brexit; if as a modern-day Ramsay MacDonald he makes Labour complicit in this historic Tory catastrophe, then expect all hell to break loose in the party. If he goes against the will of his party, the great majority of his voters and his MPs, the party will break apart – and so it should. But that hasn’t happened. These seven are jumping a gun that may never be fired. They will find it cold out there. I know that from experience. So deep was the SDP/Labour split that sitting in the Guardian canteen, Labourites picked up their plates and walked away from us SDP-ites. Schism is bitter and personal. And in this particular case, needless.

• Polly Toynbee is a Guardian columnist

• This article was amended on 22 February 2019. An earlier version said the Gang of Four broke away from Labour to form the SDP because they “would not stand on [Michael] Foot’s manifesto that pledged to leave the EU and Nato”. The breakaway was in January 1981 whereas the election manifesto under Foot’s leadership came in 1983. It sought to abandon nuclear weapons, not quit Nato.