Imagine a British election that’s not – first and foremost – about Brexit. Given the one we’re having right now it’s tough, isn’t it? There are younger voters, now in their early 20s, who have gone to the ballot box a handful of times. They will have never known an election when British politics isn’t operating under the existential question: having voted to Leave in June 2016, will the UK now leave the European Union?

If the Tories win in December then the London mayoral election of spring 2020 could well be the first such major electoral contest since the referendum not to take place under that long shadow. Polling conducted by the Mile End Institute and YouGov demonstrates what’s changed since 2016 in the London, and what’s stayed the same.

There is a neat symmetry to this, as the last contest in May 2016 saw Sadiq Khan – himself in one sense a relic of an older, pre-Brexit Ed Miliband-era-Labour politics – elected in the capital on a landslide. In the first round of voting, Khan took 44% of the votes, a 9-point lead over the Conservative Party’s Zac Goldsmith on 35%. The next nearest challenger – Siân Berry of the Greens – trailed on 6%. The voting system then whittles the candidates down to a final two: in that round, Khan beat Goldsmith 57% to 43%. 

Now, according to the polling, Sadiq Khan goes into the campaign with a commanding 22% lead over his Conservative challenger, Shaun Bailey.  But it’s actually far from clear that the final run-off will even be a Labour-Conservative battle this time round. The contest has been disrupted by jostling for position among those in third, fourth and fifth place and by the introduction of Rory Stewart into the contest.

Stewart, who ran for the Conservative leadership on an anti-Boris Johnson ticket before being kicked out of the party by Boris Johnson, has leapt into third place as an independent candidate. Of those Londoners that have a view, more are likely to say they think the final two will be a Labour-Stewart, rather than a Labour-Conservative, contest. 

At first glance this is odd, for two reasons. Firstly, Rory Stewart is not really a Londoner, and until December represented the Cumbrian constituency of Penrith and the Border. When asked, some 80% of Londoners say that it is important that the mayor has spent most of their career working in the capital city. Second, because in a city that was the only counting area to vote for Remain and has seen millions on the streets campaigning for it, Rory Stewart has long said that he supports the result of the 2016 EU referendum.

So where does his support base come from? Stewart’s 13% appears to have been taken, principally, from those that are not Tory-leaning voters. If you normally vote Lib Dem, you are more likely to support Rory Stewart than if you are a Conservative voter. The introduction of Stewart has seen support for the Green Party candidate fall by more than half, from 16% to 7% – despite clear support among Londoners for the aims (if not the methods) of the Extinction Rebellion movement, with voters in the capital saying they support them by two to one. 

All this suggests that Stewart’s progress could become a self-fulfilling prophecy. Voters in London have to cast a second preference, and if they think a Khan vs Stewart run off is likely then the independent candidate starts with broad support from voters of the parties that look set to drop out. And those who support his former party, the Conservatives, in the capital who take a different and more positive view of Boris Johnson, will be faced with a choice: cast a preference for Stewart or the Labour incumbent Sadiq Khan. 

Yet, despite all this, Khan remains the firm favourite. The reason for this is Brexit. The London mayor’s areas of devolved responsibility have little if anything to do with the UK leaving the EU, and what happens with Brexit will be decided in Westminster and Brussels rather than in City Hall. Yet Khan has staked his political chips on being a frontman for the image of London as not just the capital of the country but the capital of a place some call Remainia.

As a result, the London mayoral election will be a test case of something different: whether despite Brexit beginning to recede from view – with discussion of trade talks relegated (at least for a while) to the inside rather than the front pages – the political identities that have been created by Brexit continue to be the driving factor that determines British politics for some time to come.