Will the public take Boris Johnson seriously in 2015?

Datatable: Per cent of party leads in...

Two things stand out from these figures. The first is that Labour’s lead is slightly, but consistently, lower when the names of the three main party leaders are included in the voting intention question. This probably reflects Ed Miliband’s poor personal ratings. Some people who say that they back Labour have second thoughts when reminded that they would be voting for a Miliband-led government.

Secondly, and more relevant to this article, is the Boris effect. It seems to have almost disappeared. For the past two years, Labour’s lead has declined sharply when voting intention was asked in the instance of Boris replacing Cameron as Tory leader. But in our latest poll, it makes only a tiny difference. London’s Mayor does not seem to be quite the crowd-puller that he used to be.

This may have little to do with him. One explanation is that Cameron’s personal standing has recovered to some extent, along with Britain’s recovery. Perhaps the "Boris effect" in past polls has been more an "anti-Cameron effect", and it is this that has diminished. Even so, disappointment may beckon for anyone who expects electoral riches to fall into the lap of the Conservatives simply by anointing Boris as leader when Cameron steps down.

To explore the Boris factor in more detail, we tested six traits, asking whether they applied to Cameron, Johnson and two other possible contenders to succeed Cameron as party leader – Theresa May and Michael Gove:

Datatable: Per cent who say that they...

The good news for Boris is that he beats both May and Gove on all six (albeit very narrowly on whether he would be good in a crisis, when he leads May by a single point.) The less good news is that, while he leaves Cameron far behind on the "soft" qualities of being interesting, genuine and in touch, and holds a small lead on being seen as honest, he lags the Prime Minister on the two "hard" qualities of being up to the job of governing Britain, and being good in a crisis.

To some extent, an incumbent PM is always likely to have an advantage on these "hard" qualities, for he has plenty of opportunities to decide big national policies, negotiate with foreign leaders and send British troops into action. Boris’s big decisions as Mayor of London, such as rearranging the congestion charge zone and giving us Boris bikes, do not belong to the same league. True, Boris has weighed into big arguments about immigration, Europe and the future of Heathrow, but has had no power to implement his ideas.

Given his undoubted charisma and his way with words, he has the potential to be a big vote winner for the Tories. But, and it is in important but, voters who regard humour and a cavalier style as an asset in a city mayor with few real powers might seek different qualities in a national leader. Last week, in an interview with the Sunday Times, he talked about how his six years as Mayor had given him the administrative experience that would stand him in good stead in national politics. He has a point. But if he is to be a real vote-winner for his party on the national stage, he needs more. He needs to get serious: to show that he has the gravitas and judgment to steer Britain through the troubled waters that the country is likely to face for some years to come.