In their first poll conducted fully after all the party conferences, YouGov once again tested what difference replacing David Cameron with Boris Johnson would have on the Conservatives' poll rating. As in their previous two attempts in September, YouGov's numbers show Boris narrowing the gap to Labour by seven points: with Cameron as leader, the Tories trail by nine (33-42); with Boris, they're just two behind (38-40).
Interestingly, Boris doesn't do any better among 2010 Tory voters than Dave — both retain 65 per cent of them. What the Mayor of London does is attract more 2010 Labour voters (6 per cent of them, to Cameron's 3) and Lib Dem voters (18 per cent, to Cameron's 10) than the Prime Minister. This is reflected in the fact that, when asked who would make the better leader, 2010 Tory voters chose Cameron over Boris by 44 to 35, whereas the electorate as a whole narrowly favours Boris by 36-34.
As Sebastian noted last month, the Boris bump wouldn't be enough to prevent a Labour majority in 2015. It's also worth bearing in mind that hypothetical polling has its drawbacks. Anthony Wells has pointed out that it essentially asks people to guess what a Boris-led Tory party would be like and whether they would support it. The Boris bump may also be a case of 'the seaweed is always greener in somebody else's lake', which would disappear if he were actually leader.
Nonetheless, that seven-point boost could be the difference between winning and losing for around 50 Tory MPs. That's significant, as it would only take 46 current MPs believing they might be among them, and deciding that saving their jobs is worth Cameron losing his, to bring about a vote of no confidence.