There is something strange about the speculation over Boris Johnson's future.

David Cameron is only 45. He leads Ed Miliband as Britain's preferred Prime Minister; and though the Conservatives lag Labour by around ten points in YouGov's daily tracking surveys, their deficit is smaller than normal for governments in mid-term. There seems to be no obvious reason why a vacancy for the Tory leadership should arise any year soon.

Yet the commentariat is gripped by Borismania. Their favourite sport is to debate Johnson's intentions. Will he make a run for the Tory leadership if the party loses the 2015 election? This question prompts another. As he will have to be a Member of Parliament to stand for the leadership, will he seek a seat to fight in 2015 - or even before, if a juicy by-election presents itself? However, he would have to break his promise not to return to Parliament until May 2016, when he completes his current stint as Mayor.

Nevertheless, the speculation persists. So YouGov has explored the issues in surveys for the Sun and Sunday Times.

First, we repeated a pair of questions we asked three months ago, when Boris defeated Ken Livingstone for the second time. We asked how people would vote if the three main party leaders were Cameron, Miliband and Nick Clegg, and then asked how they would vote if the Tory leader were Boris rather than Cameron. In May it made no difference. Now it does, as these figures show

Voting intention if...

As those figures show, Labour's lead is reduced by six points when the current party leaders are named. (The first column shows our conventional voting intention figures, when we list the political parties but not their leaders). This chimes with the fact that more people want Cameron rather than Miliband to be Prime Minister. This is not surprising. It's a moot point which version of voting intention is the better predictor of the eventual election results. Much will depend on how the leaders acquit themselves during the election campaign itself.

What is clear is that, at the moment, more people say they would vote Conservative if Boris rather than Cameron were Conservative leader. The party gap closes by another five points, to just one percentage point.

However, other questions seem to contradict the suggestion that voters rate Boris more highly than Cameron:

Only 36% of voters think Boris is well suited to being Prime Minister. Cameron beats him easily, with a score of 46%. Among Tory voters the gulf is even greater: Cameron 90%, Boris 61%.

When asked directly to compare the two men, just 30% think Boris would make a better Tory leader than Cameron, while 33% think he would be worse.

How then do we reconcile the voting intention figures, which look good for Boris, with the other data, which don't?

I believe we are seeing a classic low-season verdict. Voters' attitudes are apt to vary, sometimes sharply, between high-season periods, of which the most intense are general election campaigns and low season periods when no elections are in the offing. Just now politics is about as low-season as it can get. Not only are politicians not seeking our votes, but politics scarcely features in the news at all. The recession? Bankers behaving badly? Public service cuts? Eurozone woes? Last Wednesday, Boris getting stuck on a zip wire in a London park generated more news coverage than these four sagas combined.

The key point is that when politics is in one of its low-season lulls, normal people have better things to do than fret about what ministers are getting up to - or work out how to give consistent answers to a series of poll questions. At these times, visibility is the greatest political asset; and politicians get noticed when they are being non-political. What could be a more glorious time for a politician to attract attention than a self-mocking, publicity-seeking, unpredictable Mayor of London during his city's Olympics?

That's not to say our figures should be dismissed altogether. If Boris does harbour dreams of setting up home one day in 10 Downing Street, the Olympics give him an ideal start for his own personal marathon.

But that's not the same as saying that he will do better than Cameron when we enter the high-season politics of an election campaign. Even now, when Borismania is at its height, our data suggest that voters' enthusiasm is tinged with doubts. Those doubts may well multiply when their minds are on the future of Britain rather than its yield of Olympic medals. Will they then decide that Boris is the right man to grow our economy, protect the NHS, deal with foreign leaders - and have his finger on the button of Britain's nuclear weapons?

See the full details of YouGov's most recent Sunday Times poll

See the full details and results of YouGov's latest leadership poll

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