Keir Starmer’s early lead among party members in Labour’s leadership election caused more surprise than it should have. The shadow Brexit secretary was already popular among many members for his pro-remain stance and general air of competence; the fact he is not obvious electoral anthrax helps to make him seem a safe harbour in the wake of the Tory tsunami that swept Labour away last month.

The bulk of Labour members are not hardcore Corbynites: their shorthand designation would be “soft left”. They backed Jeremy Corbyn largely due to the paucity of alternatives willing to oppose austerity in 2015. After a general election in which triangulation under Ed Miliband had been seen to lose a winnable campaign, many felt they may as well stand up for their principles and die on their feet, not their knees.

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Now the Tories have secured their biggest majority in more than three decades, that judgment has shifted. Not that Starmer hails from the party’s Blairite near-past – he is anti-Brexit and at least broadly anti-austerity. And his chances have benefited from Rebecca Long Bailey’s ill-judged campaign launch, which was laden with vacuous phrases and empty nostalgia. It is early days, of course, but this leadership election may see members opt for what they see as pragmatism over principle.

But pragmatism that departs from principle is not a purely paper exercise. It is true that nobody gains anything from a party that’s not in power. But many members will want to be able to trust the new leadership will “do the right thing” on key issues; triangulation in opposition surrenders the definition of what is politically and morally acceptable, while in government it has a human cost.

This will be tricky ground for both Starmer and Labour to navigate. The left will set Starmer numerous tests to chip away at members’ trust in him – tests of purity or principle, depending on your perspective. Austerity broke the mainstream leadership candidates in 2015 with Harriet Harman’s infamous “Labstention” on Tory benefit cuts. This time, virtually all candidates will agree on the need to end, and to varying degrees reverse, austerity. But will the party debate choosing between broad tax rises or ditching certain spending pledges? And how will Starmer handle questions on the 2019 manifesto’s nationalisation agenda?

Then there is the constitutional question. Advocates of constitutional reform unfurl a long list of magic bullets that will supposedly solve Britain’s political malaise – devolution, electoral reform, an elected second chamber and so on. These are, apparently, issues at the forefront of voters’ minds. Should Clive Lewis or Lisa Nandy make it to the members’ ballot, expect them to drive these on to the Labour agenda.

But the biggest dividing line may prove to be immigration – which, unlike austerity, is not subject to a broad consensus in the Labour party. Assuming the Tories end free movement for EU citizens, should Labour pledge to restore it? Should similar rights then be extended to non-EU migrants? What specifically does it mean to extend free movement, as Labour’s conference policy mandates? It is hard to see open borders as a winning strategy under the current electoral map; it is even harder to morally justify restricting economic opportunity (and environmental safety) on the basis of accident of birth.

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The unenthusiastic attitudes towards immigration among voters outside Labour’s city heartlands, and the sensitivity of the issue among members, makes this a big potential battleground in the leadership campaign. Both Starmer and Long Bailey will seek an as yet unknown way to harmonise the preferences of Labour members and the British public. The party’s socially libertarian left views Nandy’s campaign with suspicion on this issue. Lewis – perhaps the only candidate who’ll be totally at ease discussing free movement – has an interest in ensuring his rivals are pinned down to more specific positions than “progressive patriotism”. It won’t be pretty, but perhaps it’s time Labour worked out its view of immigration and migrants’ rights once and for all.

Whoever becomes leader faces an appalling electoral map. The Conservatives did not gain Labour heartland seats by small margins. Sedgefield has an 11% Tory majority. In Scunthorpe it’s 17% and in Grimsby it’s 22%. Mansfield – Labour-held until 2017 – now has a 33% Tory majority.

Regardless, recent decades show us that parties rarely lose power without a recession on their watch. If there isn’t one before the next election, the Tories are surely safe; if there is, their new voters may feel betrayed. Labour members will use this leadership contest to try and initiate a path back to power. The coming months will see a battle over what the party is prepared to sacrifice along the way.

• Chaminda Jayanetti is a journalist who covers politics and public services