David Cameron probably wishes that, like most people, he had never heard of the 2nd Earl of Guildford, otherwise known as Lord North.

The Prime Minister's 18th-century predecessor didn't do such a bad job, by the standards of his day, but he did make a tiny error. He was the man who "lost America".

In point of fact, North ceded the 13 Colonies to settlers who actually lived there, but at home his contemporaries didn't see things that way. Up at Edinburgh Castle there's an old door on which is carved the image of a scaffold and the words "Lord Nord": you get the idea. If Scotland votes Yes Mr Cameron's Tory colleagues will be less crude, but the punishment will be just as certain.

There is no chance that impeach­ment - the fate that befell North - will be demanded. A leadership challenge in the modern style is the likely out­come. According to some reports, that is already being discussed on the Tory back benches. For all the bland noises from Better Together, the talk is of catastrophe, humiliation, and a predicted "flood of anger".

It's nice to be taken seriously, for a change. In any case, the chatter reflects reality. How can you continue as the Prime Minister of a United Kingdom that has ceased to be united? It speaks to the job description. It also asks a fundamental question of Mr Cameron's competence. If you can't keep Britain together, what can you do?

Perhaps for that reason, among others, the Prime Minister will have none of it. This week he told Radio 4's Today programme "emphatically" that he will not quit if Yes wins a majority. The issue at stake, he said, is "not this prime minister or that prime minister, or this party leader or that party leader. What is at stake is the future of Scotland…"

Translated, this was Mr Cameron attempting, yet again, to prevent the vote becoming a referendum on the Tories. He has a healthy appreciation of how that experiment tends to end. He has been less than prominent in the independence argument for the self-same reason. He has stayed out of it, for the most part, and avoided provoking the Scots while Labour has done the front-line work and suffered the - now considerable - damage.

Ironically, what must have seemed like tactical cleverness is now the heart of Mr Cameron's Lord North problem. If and when it all goes wrong, his unruly back-benchers will ask what became of Dave when it mattered most. They will say he took refuge behind the human shields of a Better Together campaign they derided. Most of all, they will say he was cavalier in discharging a solemn duty to the UK.

A lot of those preparing to do the talking are not much interested in what becomes of Scotland. People who have expended rhetoric on the topic of whingeing, dependent Jocks are not especially sincere when they invite Scotland to stay dependent. They'd rather take a sledge-hammer to Barnett and budgets. Failing that, they will take a Prime Minister's scalp .

A Yes vote would just be another excuse to attack Mr Cameron for the sake of their real obsession, namely Europe. The catastrophe of independence - and so forth - would be an ideal opportunity to remove an obstacle, as they see it, on the road to their own referendum project. Getting the faintheart out of Downing Street in order to get the residual UK out of Europe sounds, for these Tories, like a double bonus.

Still and all these people, and many more besides, would have a point. It is unthinkable that Mr Cameron could cling on after a Yes vote. It is silly, if understandable, that he should pretend otherwise. But even on a personal level, it is hard to see how the Prime Minister can claim he would be "heartbroken" by a Yes vote yet determined to stay in his post.

The British state would be in crisis. His reputation, like his economic and defence planning, would be in ruins. Westminster would be facing the kind of upheaval it has not experienced in three centuries. It would be the sort of situation in which calls for the Queen to intervene would no longer sound quaint. And Dave would soldier on? That's risible.

It also does no service to the voters of England, whatever the "tactical" thinking. Most of what they hear about the referendum turns on explanations of how much it matters to the entire UK. This is Scotland's choice, as Mr Cameron himself would put it, but the consequences of Yes would be profound for England, Wales and Northern Ireland. We are supposed to believe, nevertheless, that the consequences wouldn't be too profound for Mr Cameron.

An English voter - or a Scottish No voter, come to that - is entitled to ask a basic question about all of this. If Mr Cameron refuses to pay the price for losing Scotland, in which circumstances would he resign? The guesses would make for an excellent parlour game, but they would not reassure anyone asking if their government is in safe hands. The evidence would be stacked high against the Prime Minister.

In fairness, Alex Salmond has also felt it necessary to say he has no intention of resigning if the vote goes against him. In this case, the mantra runs that he means to see out his term as First Minister, that he was elected in 2011 - handily, too - with a mandate to serve for five years, and serve he will. Marking a decade of his SNP leadership partnership with Nicola Sturgeon this week, Mr Salmond made his case again. Clearly, he can't be forced to quit unless his party turns against him or, by some miracle, two-thirds of MSPs force an election.He remains a popular First Minister with a working majority.

Besides, Mr Salmond and Mr Cameron are at one, for once: "It isn't about them". But the SNP convener has spent his political life working for this referendum. Ms Sturgeon is a better than able successor in waiting. Would the First Minister really want to continue?

He reckons, as you would expect, that the question won't arise. Mr Cameron can no longer muster the same confidence: hence the growling from his back benches. Mr Salmond is also spared the prospect of the kind of Tory interrogation that will be inevitable if there is a Yes vote. For the Prime Minister, it could happen even if the vote is No.

Mr Cameron has worked hard to prevent a Scottish plebiscite on his party. His own Scottish leader, Ruth Davidson, has even been obliged to suggest that her Prime Minister won't be around for much longer. She has implied, in so many words, that if Scots have no taste for the paramount Tory they needn't worry their heads about it.

That isn't really the problem. The fact that Britain's Prime Minister has felt unable to lead a campaign to save Britain is one explanation of why a referendum is happening at all. Mr Cameron's blithe refusal to contemplate resignation on September 19 is another.