Is the Labour party in imminent danger of splitting? In a Guardian interview, the leadership challenger, Owen Smith, says it may if Jeremy Corbyn remains in charge.

On Radio 4’s Today programme, the shadow chancellor, John McDonnell, dismissed the warning as misguided. “There is no way I am ever, ever going to allow this party to split.”

Who’s right? Throughout the Corbyn ascendancy I have ruled out split talk as both shortsighted and improbable. Labour split badly after the economic crisis of 1931 and again when Roy Jenkins’s “Gang of Four” broke away from Bennite Labour to create the SDP exactly 50 years later. It didn’t do either faction much good, as a repentant Polly Toynbee admitted here.

Quite right too. “Both a crime and a mistake,” as I called the split at the time. Weightier Labour figures on the Labour right like the ex-communist Denis Healey, Roy Hattersley and many others stayed on and fought their corner. Under leftist Neil Kinnock’s brave leadership, the electable coalition within the party’s ranks wrested back control, enabling Tony Blair to win three successive elections for progressive politics.

I realise that the above is a hotly disputed narrative, not least among those leftwing idealists (that label will be disputed too) who were expelled or marginalised on the march back to power. Amazingly some of them, not all Trots and headbangers, are still around and back on board. I bumped into one expellee, an old pal, in the pub last week. Within minutes he was explaining how well things are going for Jeremy. This time it’s different, he thinks. He always does, as hopeless as ever.

So I’m with McDonnell on the strategic point. Labour shouldn’t split and Smith, a newcomer to politics at the highest level, is making a mistake in suggesting it. So are his well-meaning allies in the parliamentary Labour party (PLP) whose handling of the post-Brexit crisis has been poor. “Fight, fight and fight again to save the party we love,” was Hugh Gaitskell’s response when the unilateral nuclear disarmers won a conference vote in 1960. Gaitskell was right, his acolyte, Roy Jenkins, wrong in 1981. Even though the unilateralists are now in control of the party in 2016, it’s still wrong.

Why? Because splits are a bit like divorces, usually worth avoiding if possible because of the damage they do to most, if not all, concerned. Even in its current Brexit trauma the Tory party has not split – it last disastrously did so, on the same “free trade” issue, corn law reform, as long ago as 1846. Instead it has disposed of its leader and moved on with characteristic efficiency.

Those in doubt need only look at Ukip, back in split territory again after the frontrunner Steven Woolfe MEP, the articulate but high-risk barrister/business candidate from the back streets of Manchester’s Moss Side, was barred (on a deadline technicality) from standing in the contest to replace the autocratic Nigel Farage, at least temporarily, given Farage’s Frank Sinatra tendancy to make comebacks.

I wrote about the habit of small parties run by charismatic autocrats like Farage last year and don’t repent a word of it. Big parties are like solidly built houses in an avalanche or tornado, I wrote: they still stand when shoddier constructions tumble.

So McDonnell is right to warn against splits. He’s right too to say – as he did on Radio 4 today – that he and Jeremy are “not in favour of deselecting MPs … we are going to hold this party together.” He may even mean it, but it won’t prevent a lot of trouble from old score-settling lefties, assorted hotheads and wet-behind-ears kids at constituency level who see boundary changes as a chance to purge their local members. We saw this in the 1980s and so did John.

If Jeremy wants to stop it he will have to show much more leadership than he has so far, which – apart from unavoidable and unhappy sessions of PM’s question time in the Commons – largely seems to consist of talking to admiring audiences who agree with his Latin American approach to the politics of street protest rather than power. As Corbyn’s baleful contribution to the Brexit debate (or was it cynical, as some claim?) shows, he doesn’t do outreach.

Yes, I realise that’s a tendentious interpretation too, but some people are easily misled by enthusiastic crowds. I watched one such crowd greet Corbyn in Nottingham last August and realised he was going to win, but I also noted its unrepresentative character (middle-class, public sector professionals in a university city) and wondered where Corbynism might end up.

Don’t take my word for it. In recent weeks Richard Murphy, scourge of the tax cheats and a star of Corbyn’s Nottingham rally, has given up on Project Corbyn’s lack of drive, coherence and vision. Disappointed that his call for a “People’s Quantantive Easing (QE)” has been pushed aside? Perhaps, though McDonnell and Corbyn’s latest call for £250bn worth of borrowing – to leverage another £250bn for infrastructure projects from the private sector – sounded Murphy’s kind of talk.

In any case, it’s not just Murphy, is it? Before Thursday’s 10-point Corbyn plan (and first head-to-head with Owen Smith in Cardiff) Prof Prem Sikka, another good man in the arcane ways of tax fraud and avoidance, has written a much more upbeat version for the Guardian. But Sikka is an accountancy specialist and lacks the authority of Danny Blanchflower, who made a devastating contribution to the debate the other day.

Hard to dismiss such a distinguished adviser, who preceded John McD on Radio 4 and said he doesn’t have a clue what Labour’s economic policy is. “Danny, pick up the telephone and let’s have a chat about it,” replied the shadow chancellor, but it sounds as if the economist has been no more in the loop than Angela Eagle.

All this may reflect poor organisation and lack of staff or resources. It probably does. But it also points to an introspective cast of mind which is more concerned with capturing/controlling the party – the movement, as romantic aficionados put it – than with persuading a truculent British electorate that Labour is making constructive proposals, ones it has more than a cat’s chance of delivering in office.

And that’s the point really, isn’t it (cries of “No”)? Thursday’s YouGov poll suggests support for either wing of the Labour party would sink even further – see here – if it split again, as happened in the 80s when the Bennites, with Jeremy Corbyn’s support, were trying to do to Foot and Kinnock what they deplore today in Owen Smith.

As Blanchflower points out, most polls are awful: 95 out of the past 100 show Labour behind despite Tory disarray and Brexit strategy collapse. Without the support of his MPs and credible policies, “the bond and equity market would eat him for lunch”, as the prof puts it. Contrary to Len McCluskey’s dark musing, MI5 doesn’t need to plot. The party is marginalising itself.

Still not troubled? Owen Jones, no faintheart, is troubled here. Julian Baggini explained the perils of simple-minded populism – it’s not just about us in Britain – here. And this Marxist analysis – it’s long but thoughtful – condemns what it calls “the terrifying hubris of Corbynism”. They’re all weightier than Richard Burgon or Diane Abbott on a good day.

Is Owen Smith the answer? For the first time in 40 years of watching Labour leadership struggles I have to admit I don’t really know him. He’s made some good points and said some silly things clumsily. I’m not convinced, but I haven’t been by the wannabe leaders on offer since Ed Mil knifed his brother with Len McCluskey’s help in 2010. I share Blanchflower’s doubts that Labour loses when it’s not leftwing enough.

In standing when it wasn’t his turn Smith has at least showed leadership instinct. Leaders don’t have to seek permission to step forward (in Jeremy’s case, pushed forward with an “it’s your turn” from McD). They hear the call and do it. Whoever wins has to compromise with the loser – as Hillary Clinton and Bernie Sanders are awkwardly coming to terms – and, more important, compromise with a sceptical electorate which doesn’t do rallies or marches.

If Theresa May proves to be any good and the economy staggers along with less austerity under Phil Hammond, the 2020 election may already be lost.

But politics is a long game and adaptable young politicians learn to adapt and fight again. When Corbyn says he’s determined to stay because he has the backing of the movement, he may even mean it. So the bulk of the Labour movement, which doesn’t want to wait until 2025 to win power, must make it clear to Jez and the purists that they must adapt or go. That means a healthy vote for Owen Smith. The ball is in the members’ court, but members need to remember they are only playing in the qualifying rounds. The cup final, the battle for real power, is decided by voters.