Voters may have something to say about Cameron's hope

Satire packs its bags and calls it a day when faced with that one. A third term? Even Tory backbenchers can spot a deviation from known reality. Mr Cameron did not win his first term outright; he will be fortunate indeed to do any better in May. To begin to dream aloud of what the future might hold for a 53-year-old in 2020 looks - as every opponent has pointed out - like presumptuous arrogance.

In fact, it looks worse than that: it looks daft. It looks like a specialised form of stupidity. To announce at the ground zero of the Cotswolds kitchen supper that you have adopted the Shredded Wheat programme for career development - two bites are enough - is beyond baffling. It makes no sense at all.

This week Parliament will be prorogued; dissolution will be announced on Monday; the deadline for nomination papers falls a fortnight on Saturday: even by official measures, the general election is upon us. In that race, most polls have Mr Cameron's Tories trailing slightly with the electoral system stacked against them. So what's the smart move? Act as though the election is in the bag while encouraging a blizzard of speculation over your leadership?

Presumption isn't the half of it. Bizarre as it might sound, the third term Mr Cameron dismisses so airily would make him the longest-serving Prime Minister since Pitt the Younger (almost 19 years) and Walpole (almost 21 years). To paraphrase: I've read about young William Pitt, hero of the Napoleonic Wars. Mr Cameron, you're no Pitt the Younger.

It seems we are to be spared the comparison. Graciously, nevertheless, the Prime Minister seems to take a full decade more or less for granted. Nothing hinders him from "going on and on" save the risk of eventual madness - naming no names - but Dave will be satisfied by then. Lasting longer than Asquith, Churchill, Wilson, Baldwin, Disraeli and Macmillan will do for him. He will probably - but you never know - allow Tony Blair the edge by a couple of months.

That name might provide one clue as to what is going on in Mr Cameron's mind. He has never hidden his not-so-grudging admiration for Mr Blair. Telephone conversations between the two men have been frequent enough to warrant comment. Reportedly, the Prime Minister once described himself as the "heir to Blair" at a meeting with newspaper editors. In many respects he has modelled himself on his predecessor. So why not match the former Labour leader's record?

It sounds like a trivial ambition, but all prime ministers become obsessed sooner or later with their place in the history books. Even Mr Cameron's declaration (to the BBC's James Landale) that he needs two full terms to "finish the job" speaks of an ego in full working order. Yet great prime ministers - Attlee, Lloyd George - had no such luxury. The idea that a decade in power is the mark of success is a modern conceit. Fifteen years needn't be ruled out: it's nonsense on any terms, bad for the barking mad office-holder, for Parliament, and for country.

Mr Cameron's allies would have us believe he has understood that much. If you believe them, he is doing us all a favour by showing how sane and modest he is in settling for a mere decade. That doesn't explain the strategy, if any, behind his declaration. There is no obvious political cost-benefit analysis that works in his favour.

You might argue that a handy distraction was created. For one thing, the man from the BBC seems not to have asked Mr Cameron what would follow if the Tories are slung out in May. The Prime Minister's resignation in that highly-likely event is taken for granted - he has said he will stay on as MP for Witney if voters "hoof out" his party - but a direct question would have brought the leadership issue into focus. Mr Cameron has postponed the matter, so he might hope, until 2020.

If so, it's less a hope than a delusion. All attention now is on George Osborne, Theresa May, and Boris Johnson. The fact the Prime Minister even named these three as "great people coming up" will seem high-handed to some in his party. The fact he nominated Alexander Boris de Pfeffel Johnson, a character still lacking a seat of his own, was either cunning, careless, or crass.

The fact remains the Tories embark on an attempt to cling to office with a leader who has announced his retirement plans. For a bonus, they have three prominent potential successors who will calibrate every word and deed to the demands of a future contest. Mr Johnson has been playing that game for years. Mr Osborne and Ms May have to decide if they are helped or hindered by their service to Mr Cameron. His musings haven't exactly helped.

If Lynton Crosby, the Tories' Australian "campaign consultant", came up with this one, he could have some explaining to do in a few short weeks. Equally, if the alleged master strategist allowed the party leader to unburden himself so casually - to a reporter, and on camera - he will have broken one of the cardinal rules of his dispiriting game. If there is a plan at work it's so cunning, in the words of Blackadder, you could stick a tail on it and call it a weasel. And in the Cotswolds they'd shoot it.

It's possible, just possible, that Mr Cameron and his advisers are straining after an old Harold Wilson trick. The otherwise comical notion the Tory leader can pick and choose between two and three terms in Downing Street might be an attempt to create the impression his is "the natural party of government". The old Labour trickster got away with it in a conference speech in 1975. The claim was far-fetched in that of all years, but somehow it worked.

The suggestion would fit with Mr Cameron's refrain of a job half done, a deficit reduced by half, and a "long-term plan" that needs - according to Mr Osborne - another five years. But all prime ministers and parties talk this way. The great programme is never quite complete. The difference is that most prime ministers do not chat about possible successors when there is an election to fight.

Profiles of Mr Cameron often include a paragraph or two on his breezy Etonian self-assurance. He has been lucky enough to be able to take a lot in life for granted, a political career not least. But with this bit of business - surely the first authentic "gaffe" of the campaign - the Prime Minister has taken the electorate for granted. While he contemplates his right to a third term, never mind a second, they have an answer: We'll see about that.