Unity is breaking out in the Labour party as the leadership election rolls on. Or, at least, so it keeps being said. There is unquestionably some evidence. It is partly embodied by the growing consensus, which has surprised many, that the contest is now Keir Starmer’s to lose. But the unity talk is also partly wishful. It would be wise not to take it entirely at face value.

Unity is mostly better than its opposite. After Labour’s brush with electoral mortality in December, a search for unity obviously makes immediate sense. This is especially wise for a party that throughout its history has mostly been a federation of interests and ideologies rather than a cult restricted to the doctrinally pure or the wretched of the Earth.

But unity in adversity is not the same as the pursuit of a unified project. Of course, unity is strength. A house divided against itself cannot stand. All that stuff is true. But unity is not everything. In some circumstances unity can merely paper over cracks where rebuilding is needed. Labour’s long history is also full of decisions to banish members from even the broadest tent.

The real question facing Labour about unity is this. How can it end the deep and disabling internal divisions of the past five years and converge behind an agreed set of electorally sustainable reformist and social democratic objectives? This question underlies the 2020 leadership election. It has no single magic answer. Instead the answer lies in a variety of interlocking responses to the dead end into which Jeremy Corbyn led the party.

Here are two responses, though there are many others. The first is the need for a serious appreciation of the electoral geography that will face Corbyn’s successor. This is important because of the variety and complexity of the fractured country Britain has now become. It should have been the factual bedrock of the leadership contest.

This transformed electoral landscape goes far beyond even the daunting gulf between the Conservatives and Labour. All Labour members should commit to memory the fact that, since 12 December, Labour now needs 124 extra seats to win, more than a hundred of them now held by the Conservatives – a proportionate increase of more than 60% that no party has ever achieved in one election in modern times. This task far exceeds even the difficult task of regaining the so-called red wall seats of the north and Midlands.

That’s because the structure of Labour’s vote has changed and is still changing. In the past, Labour’s safest seats were in the coalfields and heavy industrial areas. Never again. Now they are in cities, suburbs and university towns – except in Scotland – and areas with significant ethnic minority populations. To win, Labour doesn’t just need to rebuild its red wall. It also needs to win places such as Basingstoke, Hexham, and Uxbridge and South Ruislip (Boris Johnson’s seat), which have never elected a Labour MP, even in 1997. According to the former Labour strategist Greg Cook, who is not one for hyperbole, “this new electoral landscape is arguably the most formidable Labour has ever faced”.

Labour’s second problem – which relates directly to the shift in electoral geography – is its political narrative. Here’s where things get more difficult as a direct result of the Corbyn legacy. But, right on cue, a possible guide through these troubled waters has emerged. In his richly nuanced Warring Fictions, the most stimulating book I have read on Labour in ages, party activist Chris Clarke highlights three legacy myths of the Corbyn era’s left populism that Labour needs to overcome.

The first, which he dubs the “dark knight” myth, claims that politics is a Manichean struggle between the morally good in-group (in this case the Labour left) and the wicked and selfish centre and right (the Tories and all those who can be tarred with the neoliberal or Tory-lite labels that have become Labour’s curse). This, says Clarke, leads to a destructive exclusivist allure of identity, moral self-regard and complacency.

The second, the “puppetmaster” myth, says everything is ultimately a conspiracy against the public. In this view, the country is run by an elite of (sometimes foreign) magnates, media and deep-state agencies bent on crushing the public into submission to the status quo. As a result, incremental and reformist politics can be dismissed as hopeless.

The third, which Clarke calls the “golden age” myth, is a narrative of decline from an era of supposed virtue – the postwar Attlee era looms large – against which later eras are always found wanting. This leads to the favourite Labour trope of leadership betrayal, in which everything is a moral failure by corrupted leaders who should have acted in more virtuous ways, whatever the constraints. There is no room here to go into the details of Clarke’s argument, but they are all familiar from the Corbyn era, as well as from the parlous state of the US Democratic party.

Labour’s leadership election matters a lot, but in reality it only matters if Labour makes a break with the recent past. To signal this – a precondition for winning in Basingstoke as well as Bassetlaw – the first thing the new leader should do is to unequivocally purge the stain of antisemitism from the party. The corrosive “dark knight” idea that there can be no enemies on the left will need to change from day one.

Unity is fine and dandy, but in practice it has to be unity that engages in a principled way with the country as it is, not the mythologised country the Corbyn project would have liked it to be.

• Martin Kettle is a Guardian columnist

• This article was amended on 6 and 7 February 2020. The name of Boris Johnson’s seat is Uxbridge and South Ruislip, not Uxbridge as was originally stated. The constituency, which was created in 2010, has never been held by the Labour party, but the former constituency, Uxbridge, did return Labour MPs on several occasions, most recently in 1966. The article was further amended because an earlier version said that no party had ever achieved an increase of 124 seats at one election in modern times. In fact Labour gained more seats than this in the 1997 election. This has been corrected.