Prime ministers do not pre-announce their intention to stand down arbitrarily. On the eve of the 2004 Labour conference, Tony Blair declared that he intended to serve a “full third term” but would not seek a fourth – scant consolation to Gordon Brown, who believed that his great rival had privately pledged to step down much earlier.

Cameron’s disclosure to the BBC that he will serve a full five years if re-elected, but step down thereafter, also requires decoding. In January 2013, he told me that he wanted to stay in No 10 until at least May 2020. He has now made that election month his terminus ante quem – the date before which he will depart Downing Street.

David Cameron: I would not serve third term as PM Read more

As with Blair, the declaration is an act of both strength and weakness. It quashes the growing orthodoxy that, should he be re-elected, Cameron will step down after the referendum on Britain’s membership of the European Union, which he has promised will be held in 2017 at the latest.

Boris Johnson’s supporters had warmed to the idea of a smooth transition after a vote to stay in the EU. Cameron has poured a patrician bucket of cold water over such hopes.

That said, it is also clear that the PM has been forced to confront his own shelf-life and its consequences – doubtless in consultation with his ubiquitous pollster, Lynton Crosby.

Election 2015: David Cameron rules out third term - live Read more

Though Cameron has generally rated highly in opinion polls – and invariably scores better than Ed Miliband – the shine has evidently diminished, to the point where he has felt obliged to acknowledge his own eventual obsolescence explicitly as part of the Tories’ election offer: hello, I must be going.

Though Cameron is a passionate family man and will enjoy life after Downing Street and (if he makes it to 2020) 15 years at the helm of his fractious party, no prime minister likes to name the date after which “it will be time for new leadership”.

Mindful of an election in which humility may conceivably be rewarded, he has presented his cabinet as a team rather than a personality cult, and singled out Theresa May, George Osborne and – from outside ministerial ranks – Boris as “great people” with “plenty of talent”. Thereby, he has also signalled the start of an almighty leadership campaign that may last for five years. There will, to coin a phrase, be blood.