Either David Cameron is one of the more unusual men ever to have become British prime minister — one of those rare birds in politics (the last was Stanley Baldwin back in 1937) who quit while they are genuinely ahead — or he has dropped a bloody great brick as well as a bombshell.

Mr Cameron’s “announcement” (if that is what we can call it) that he will not serve a third term was surely an unforced error from an overly chillaxed old Etonian forgetting who he was chatting to, rather than a Machiavellian masterstroke from a politician at the height of his powers. Bombshells, when deliberately dropped, are carefully controlled explosions. This one could cause considerable collateral damage.

True, it does not necessarily do the Conservatives any harm if talk turns to the issue of leadership. After all, one of the party’s key campaign themes is “the choice” not just between competence and chaos but between the man who at least looks and sounds like a prime minister (even if he does not always act like one) and the lesser-fancied Ed Miliband, Labour leader. Trouble is, that choice has suddenly got more complicated: would you like that Dave with a side-order of Boris or Theresa or George? Or maybe someone we do not even know about yet?

Even worse, the prime minister’s slip is a massive distraction with a big opportunity cost. There are only six weeks until the election. Time is short — and therefore precious. All the time we are talking about whether Mr Cameron was right or not to say what he said, we are not talking about all the things Lynton Crosby, the Tories’ pugnacious election strategist, really wants us to be talking about. Namely: how the party’s long term economic plan is delivering a recovery that a spendthrift Labour party — in thrall to the unions and soft on scroungers — will only wreck.

Nor does it seem smart for a leader to tell a country that, generally speaking, prefers a plucky underdog, that he is already planning his triumphal farewell tour, let alone his victory celebrations.

Presuming for a moment that none of this makes any difference on May 7 and Mr Cameron scrapes back into Downing Street, it will definitely do so after May 8. Those he named as possible successors — Boris Johnson, Theresa May and George Osborne — will be under constant surveillance for anything that looks like an attempt to get ahead of their rivals. So too will anyone else who hopes, not altogether unreasonably, that the shine might have gone off all three of them, opening the way for a younger, outside challenger. Leadership speculation is destabilising anyway. Combined with the thrills and spills of a minority government — which may be the most likely outcome — it really will be something to behold.

There will be those who argue that the move by Mr Cameron was all part of a cunning plan. Some of the prime minister’s best friends have already tried to present it as such. But if that was the case, why were Conservative spinners suddenly sent out to reassure us that Mr Cameron’s second term would be a full term — when we know it will be nothing of the kind? It takes months to run a leadership election and any new leader would want to be in the job for at least a few months more before going to the country. Count backwards from May 2020, and that means Mr Cameron calling it a day in autumn 2019 at the absolute latest.

If Mr Cameron really is in the Baldwin mode, here is the big difference. Baldwin was handing over a massive majority to a successor — Neville Chamberlain — who virtually everyone agreed on, and who in the normal course of events would have had at least two more years to establish himself. More recent history — think Gordon Brown and Tony Blair — suggests even that is not a guaranteed recipe for success. It would be overdoing it to say that what Mr Cameron has accidentally suggested is a recipe for disaster. But it may not be far off.

[Originally published at http://www.ft.com/cms/s/0/3e14263a-d217-11e4-b66d-00144feab7de.html]