If Labour are lurching left, how do we explain the rise of Tessa Jowell?

Lots of people are wrong about the Labour leadership contest. The only question is who.

Nominations from constituency Labour parties, independent polling, new membership figures and commentary from senior figures in the party all suggest that Jeremy Corbyn is storming to victory.

Yet the bookies and opinion polls of Labour supporters suggest Andy Burnham is still the likely winner on the day. Meanwhile others still predict Yvette Cooper will scrape through enough second preferences to squeeze through the middle.

So which predictions are wrong, and why is there so much uncertainty? Could we be about to face an even bigger upset than Ed Miliband's surprise win over his brother in 2010?

To understand the confusion, we only need to look at the party's London mayoral contest, where all the evidence suggests Tessa Jowell will emerge as the clear winner in the race to be the Labour candidate.

Yet how can such a staunch Blairite like Jowell be so far ahead in London, whereas her ideological opposite Jeremy Corbyn is apparently winning nationally? If Labour is, as so many commentators suggest, lurching to the left after the general election, then how can we explain the success of such a leading figure on the right?

There are several possible explanations.

1. Corbyn isn't really ahead

Analysis by Sky News suggests that at least half of all those eligible to vote in the Labour leadership contest are new members or supporters since the general election. Separate polling by YouGov suggests this group are far more likely to support Corbyn. But could this influx of newer and louder left-leaning members be skewing perceptions of the Labour leadership race? How many of those who have signed up as 'registered supporters' will actually bother to vote and how many of these will stick with their current choice? Is there a silent majority against Corbyn? And could those second preferences from the anti-Corbyn majority be enough to keep him out?

2. Jowell isn't really ahead

Jowell's camp remain confident that she will emerge as the Labour candidate. Her campaign has the kind of resources and manpower that none of the other campaigns can come close to matching. When Jowell turned up at the Evening Standard mayoral hustings this week she was followed by a larger entourage than even the current occupant of City Hall manages.

But could Jowell's confidence be misplaced? So far we have very little clear data on how Labour members in London actually plan to vote. The CLP nominations suggest she is ahead, but rules specifying that CLPs must nominate at least one woman for their candidate mean these figures could be heavily skewed in her favour. However other figures, like the large numbers of Labour councillors backing her bid, suggests she really is in first place. But how can we square this with the decades of dominance by Ken Livingstone in London. And more importantly, how can we square that with the rise of Corbyn and the non-rise of Liz Kendall nationally?

3. London Labour care more about winning

One possible answer to this disparity lies in the current occupant of City Hall. In the last two mayoral elections, Labour put up a left-wing Corbyn-like candidate and, in both of those elections, they were beaten by a right-wing Tory candidate. I have argued before that this has far more to do with personality than ideology. In a personality contest between Ken Livingstone and Steve Norris, Ken won twice. In a personality contest between Ken and Boris Johnson, Boris won twice. Yet after two such clear defeats, have London Labour members finally become more focused on winning than their national counterparts?

It's possible, but is this enough to explain such a stark divide between the parallel rise of Jowell and Corbyn?

I'm not sure. But one thing is for certain, when the final result comes through next month, many of us are going to be proven very wrong.