You know someone has lost it when they go for the person rather than the argument. For all that people think that Prime Minister's Questions is an undignified shouting match, it operates under rules, and when those rules are broken it is an indicator that something is up. What was up on Wednesday was Cameron's time as undisputed master of the Chamber. Since Tony Blair stood down six years ago he has commanded the place. Until this week.
As he tried to explain why the slightly doddery John Major didn't really know what he was talking about and that high gas and electricity bills were all Miliband's fault, his own side knew he wasn't making sense. One could feel some of that invisible power seeping across the Chamber to the other side. It animated Miliband like a jolt of electricity: he became overexcited and too pleased with himself, but it didn't matter. Cameron was in trouble.
A cynic might say that being told off by John Bercow for "unparliamentary language" was a Conservative gain for the Prime Minister, in two ways. It drew attention to the "con" phrase, which, I am told, plays well in the focus groups. "People recognise a con when they see one," a Downing Street official tells me. "They think Ed Miliband's price freeze is like a bump in the carpet: if you make it flat in that area, it just pops up somewhere else."
My source says the Tory party's opinion research suggests that people are "incredibly world weary" about Labour, that they (or the men at least) liken the party to "an old girlfriend who has cheated once too often". This is a negative business and the "energy price con" is the sort of thing that goes down well with the anti-politics hordes who think that Russell Brand is a beacon of principle.
The other way in which the Speaker's reprimand might have helped Cameron is that it united the Tory party, after a delay to think about it, in its disapproval of Bercow. Instead of mutterings about why their leader handled PMQs so badly, Tory MPs were fuming about the "Labour" Speaker. A lively debate continued in Westminster yesterday on whether the Speaker's ruling was correct. He let it go once, after all, although textual analysis of Hansard revealed that Cameron's first reference was to Miliband acting "like" a con man, while the second time was more direct: "It simply is the politics of the con man to pretend that you can freeze prices when you are not in control of global energy prices."
I think Bercow was right. Calling someone a con man is saying that they set out to deceive. That is not allowed, rightly. That is why the parliamentary rules are a good indicator: Cameron crossed the line from argument to insult. Some Tories, in a state of high moral umbrage, claimed that Bercow had allowed Labour to call Jeremy Hunt a liar, when he was Culture Secretary last year. He did not. He allowed Chris Bryant, the Labour frontbencher, to say that Jeremy Hunt lied to the House.
On that occasion, I think Bercow was wrong. He should have ruled it out of order, but it is not quite as bad as simply calling someone a liar, and the Speaker did have a reason, which was that Bryant was debating a Labour motion that alleged Hunt had failed to give "accurate and truthful information to Parliament".
However, drawing attention to the word "con" and uniting the Tory party against Bercow are not enough to compensate for the Prime Minister's humiliation in the House of Commons an unexpected and unnecessary abasement at the hands of his previously supportive predecessor, John Major. All Cameron had to do was to welcome Sir John's helpful contribution, through gritted teeth if need be, to say how much better it was than Miliband's half-baked plan, which Sir John had himself criticised, and to add that he had ordered an investigation into whether the energy companies are indeed making "excess profits", as Sir John alleged.
I understand that something along those lines was indeed what was discussed with Lynton Crosby, Cameron's election adviser who helps him prepare for PMQs. But I deduce from the disarray of the Downing Street spokespeople, as they tried to explain to journalists afterwards what the Prime Minister meant, that something went awry in the execution.
It is unclear whether Cameron intended to say he would "roll back" green taxes on energy, which made it sound as if he wanted to abolish them, and he failed to do the obvious thing of saying, as John Major did, that Miliband's heart might be in the right place but his policy wouldn't work. That is why you shouldn't insult your opponent: it is cleverer politics to accept that he or she wants the same things that you do warm and well-insulated pensioners, in this case but would go the wrong way about it.
Normally, Cameron knows this. But he seems to have been stung by Major's disloyalty and Miliband's obvious cock-a-hoopness into revealing the petulant side of his personality that he has kept suppressed for so long. He doesn't think that the energy companies really are making excess profits, because he is a rational policy wonk and there is no evidence to suggest it. And he genuinely thinks that Miliband's policy of freezing prices while "resetting the market" is populist gimmickry on jargon-filled stilts. He is right: the idea that, if you switch the market off and then switch it on again, energy will miraculously become cheaper, is silly, as Ed Balls, Peter Mandelson and Dieter Helm, the professor who used to advise Miliband in government, all know.
Ed Miliband understands that. And he knows how to engage in personal abuse without appearing to. When he spoke of "the ordinary people of this country whom this Prime Minister will never meet and whose lives he will never understand", he was slyly claiming a monopoly on compassion. It was unfair but effective, whereas Cameron's lapse into calling his opponent "a con man" was a sign of ineffective weakness. The rules of unparliamentary language tell us something about the shifting balance of power as the election approaches.
John Rentoul is chief political commentator for The Independent on Sunday