Sadiq Khan's campaign hit back strongly today at a claim by Zac Goldsmith that the Labour candidate would put London's green belt at risk.
In unusually forthright language, Khan described Goldsmith's claims as "lies" adding that "I've repeatedly said I'll protect the green belt."
On the face of it, this is true. Khan has been asked at multiple hustings, both during this campaign and during the Labour selection campaign, whether he would build on the green belt and he has always said no.
In fact with the one exception of David Lammy, there has not been a single candidate, or potential candidate, in this race who has openly called for the green belt to be built on.
So are Goldsmith's campaign "lying" by suggesting otherwise. Well not quite. Because when you look back at Khan's record as a minister, you can find multiple examples of where he has supported at least some development on the green belt.
In 2008, Khan was a local government minister. When asked by a Conservative shadow minister about the sale of allotments, he replied that there was only "finite space" which could be built on.
"He and his colleagues complain about developments on the green belt, but there is a demand for housing and finite space," he replied, adding: "Last time I checked, we were an island."
Khan also announced a consultation while a minister into the "possible selective review of the Green Belt" at Guildford and told the House of one study which showed "there may be a need for green belt amendments" in Birmingham, Rugby and Warwick.
Of course this was all a long time ago. But even during this campaign Khan has not been entirely dismissive of building on the green belt. Asked by the Economist whether he would ever consider putting more housing on the green belt, he replied that he was "committed to protecting the green belt," but added "if I was persuaded that all the possible pieces of land in London were being used sensibly and were built-upon, building on the green belt would be something we could look into."
This is important because some housing experts say there simply isn't enough available land within the city in order to meet demand. With Khan reluctant to commit to the demolition and regeneration of council estates, as Goldsmith has called for, the Tories say Khan would have to consider building on the green belt instead.
Does it matter?
When Londoners are living through a major housing crisis, the question of protecting golf courses in outer London may seem like a rather fringe concern. But the politics of the green belt could be an important factor in this race. Boris Johnson won the last two mayoral elections mostly by motivating outer Londoners to come out to vote in much greater numbers than inner Londoners. Livingstone's relatively poor showing in the "doughnut boroughs", compared to Boris was the decisive factor in Johnson's two victories. By raising fears of a "Corbyn-Khan experiment" with the green belt, Goldsmith's campaign hope to mobilise many of the same voters who won it for Boris in 2008 and 2012.
The limited polling we have seen so far suggests Goldsmith does much better among outer and older Londoners - exactly the two groups who are most likely to vote and exactly the groups who are most likely to be concerned about the green belt. Taken alone, it is unlikely to swing the election, but taken together with warnings about Khan "risking the Freedom Pass" (he won't) and raising his share of the council tax (he might) it could form the basis of a successful campaign for Goldsmith.
Khan's team are undoubtedly aware of this, which is why they have hit back so hard against Goldsmith today. Unfortunately for them, Khan's record on the green belt does not look as consistent as he has claimed.