Tonight, Labour's London mayoral hopefuls will debate in public for the first time at an event in Westminster.
One of the big issues set to come up is the expansion of Heathrow airport.
The question of whether to expand Heathrow has never been a decisive factor at any of the previous mayoral elections for the simple reason that the leading candidates have all opposed it.
This consensus has survived four mayoral elections. However, that consensus has been broken following the entry of Zac Goldsmith into the race for the Tory nomination.
Goldsmith is one of the leading opponents of building a new runway at Heathrow and has previously pledged to resign if the Tories go ahead with expansion. If Goldsmith wins the nomination then opposition to Heathrow expansion is likely to be at the front and centre of his campaign.
By contrast, the leading candidates for the Labour nomination all have a record of backing a third runway at Heathrow.
As transport minister under Gordon Brown, Sadiq Khan led Labour's support for a third runway at Heathrow. Both Jowell and Lammy joined him in voting in favour of expanding Heathrow. Only Diane Abbott voted against.
Asked this week by Andrew Marr whether she continued to support expansion, Jowell highlighted that she had voted for it but said that she would wait to see the Davies Commission report before deciding what to do as mayor.
This 'wait and see' position has until now also been followed by both her leading rivals Sadiq Khan and David Lammy. However, Goldsmith's entry into the race has changed all that.
Speaking to the Evening Standard today, Khan committed to opposing Heathrow expansion, regardless of the outcome of the Davies Commission report, and branded his rivals "ultra-Blairites" for continuing to support it.
"Tessa and David will argue Heathrow's case, but I am arguing London's case against its expansion," he said. "People must ask themselves who best represents London’s interests."
According to the Standard: "He agreed that Heathrow expansion was a 'Blairite policy', saying: "I suspect that's why Tessa and David are in favour of it. As ultra-Blairites they will have to argue why they think runway three should go forward."
The personal nature of Khan's intervention has angered his opponents, who accuse him of opportunism.
"If David is an ultra-Blairite for not opposing Heathrow expansion then so are the GMB who back expansion and support Sadiq," a supporter of Lammy's campaign told Politics.co.uk.
"This is a serious issue and we need to deal with it seriously. For Sadiq to make this kind of attack and U-turn just days after Goldsmith announced is pretty shocking."
Khan's campaign remain unrepentant.
"It's impossible to win London while backing Heathrow expansion," a source close to Khan told Politics.co.uk.
"If Jowell wins the nomination, Goldsmith will use her support for Heathrow against her and she will lose."
Opportunistic or not, Khan's decision to oppose Heathrow expansion creates a clear dividing line between the leading candidates for mayor. It also poses a big problem for Jowell.
A Evening Standard poll out today suggests the former culture secretary is still the overall favourite with voters in the race to be the next London mayor, although the gap between her and Khan among Labour voters has almost disappeared.
However, if the Davies Commission do recommend expansion at Heathrow, then Jowell's support for expansion would become a major obstacle to her actually winning back City Hall for Labour.