It really comes to something when the leader of the British Conservative party appears to want the UK not just to adopt a presidential system, but a system that is in fact more presidential than the United States itself. In the US, there is no provision whatsoever for an extraordinary general election to be held to elect a new president should the incumbent resign in office - instead there is a line of succession to determine who will see out the remainder of the normal term of office. The most famous example is of course Gerald Ford, who was the wholly unelected American president for roughly the same period of time Gordon Brown has now served as Prime Minister without "his own mandate".
A number of commentators are scratching their heads and wondering aloud whether this new "no unelected PM" policy of Cameron's really is the impulsive, back-of-the-envelope job it appears to be. But it could just as easily be that this was always a planned move, designed as a pretext for recycling tried-and-tested vote-winning jibes about Gordon Brown running away from the verdict of the electorate, which would otherwise have been impossible to sustain beyond the date for the election being set. If that is the case, it simply makes it an even more disgraceful tactic.
Even today, there are still some arch-unionists who accuse the pro-devolutionists of 'constitutional vandalism' in the late 1990s for introducing an unstable settlement for partisan gain that was always going to have to be revisited later - well, it's difficult to know how else to characterise a proposal to essentially end the UK's status as a parliamentary democracy, purely for the short-term benefit of some positive headlines during just one day of an election campaign.
And make no mistake, the effective end of parliamentary democracy (already on its last legs) is exactly what Cameron's idea entails. Once it's established in law that general elections are de facto elections for the office of Prime Minister, the principal role of parliament becomes something akin to the American electoral college, ie. simply to rubber-stamp the electorate's 'choice of Prime Minister'. And if they fail to sustain the correct person in office for a full five years, they become 'faithless electors' against whom action will be taken - ie. automatic early dissolution. The idea that what is needed to restore trust in politics at this particular moment is to demand that members of parliament behave as lobby fodder to an even greater extent is deeply depressing.
But the reason that this proposal, if ever enacted (which I doubt) would have to be swiftly revisited is that it's based on an utterly half-baked premise. You simply can't credibly complain about a new Prime Minister who takes office in mid-term being 'unelected', if you're apparently quite happy that it remains the case that all Prime Ministers - including those who win general elections - are not directly or indirectly elected. They are, of course, appointed by the monarch. Logic dictates that if the law insists upon a direct personal mandate for an incoming PM in mid-term, it must do so at all other times as well.
A true constitutional reformer would have recognised that from the start.