An election campaign is as much a battle to define one’s opponent as it is to define oneself. This is particularly true of a campaign that boils down to a straight choice between two candidates.

The London mayoral election presents a perfect case study of this struggle, as Labour’s Sadiq Khan and the Tory Zac Goldsmith both seek to forge an identity that is bolstered by their party affiliation, but not captured by it.

For Goldsmith, an MP with a track record of independence, the challenge is to capitalise on the Tory strength of economic competence without being seen as an extension of the Tory government. For Khan, the focus is on forging an identity sufficiently distinct from the current incarnation of the Labour party.

This is easier said than done, since the Tooting MP nominated Jeremy Corbyn for the party leadership and is widely seen as having benefited from the Corbyn surge to secure the nomination.

Tory strategists are desperate to paint Kahn as Corbyn’s man in London. Indeed, David Cameron has warned Londoners that Khan would make the city a testbed for so-called Corbynomics.

Given the Labour leader’s dismal poll ratings when it comes to economic credibility, this has the potential to be a strong line of attack. The problem for the Tories is that Khan is hitting back, hard.

He routinely distances himself from the more radical musings of Corbyn and his team. For example, shadow chancellor John McDonnell has openly flirted with the idea of shutting down the City of London Corporation – an idea immediately lambasted by the Labour candidate for City Hall.

Undeterred, the Goldsmith camp today attempt to tarnish Khan with Corbyn’s attack on “corporate subsidies” – which actually include vital investment incentives and grants.

The strategy is pretty clear: Tie Corbyn to Khan’s legs and hope that he sinks – or at the very least engineer stories about splits in the Labour party.

The difficulty for the Tories is that London voters already see the election of a mayor as an exercise quite distinct from the back-and-forth of national politics.

This may be to Khan’s advantage, but the truth is that the Tories are on to something. Just how independent of the Labour leadership will Sadiq Khan prove to be?