Napoleon wanted generals who were lucky. Napoleon would have liked David Cameron. He became Tory leader when Tony Blair’s electoral magic had faded and his days were numbered. Lucky Dave then fought the 2010 election against a Labour party that had been in government for 13 years and was showing its age.

His rival for the premiership had presided over the most severe economic crisis since the 1930s and, on his own account, Gordon Brown was not a politician suited for the television age. When Mr Cameron failed to parlay those advantages into a parliamentary majority, he borrowed one from the Lib Dems, who did sterling service sustaining him for five years while destroying their electoral base in the process.

He gambled the United Kingdom with a referendum on Scottish independence. Labour did the heavy lifting to keep the UK intact and its reward was to be toxified as Tory collaborators in the eyes of many Scots. The devastation this wreaked on Labour support north of the border played to his advantage at the 2015 election by allowing the Tories to scare English voters with the thought that a Miliband government would be a marionette of the Nationalists. Lady Luck also smiled on him when the pollsters, by calling the election wrong, helped smooth his path back to No 10.

I don’t put all this down to blind chance. That would be to underestimate Mr Cameron. He would not be approaching his sixth anniversary at No 10 were he not highly skilled at exploiting the opportunities that time, chance and opponents have presented to him. Like all successful leaders, he has made the most of his good fortune.

The trouble with luck is that she eventually runs out. She seemed to be bidding farewell to this prime minister last weekend. He had been hit with the most dramatic and damaging resignation of his premiership when Iain Duncan Smith quit the cabinet in a fit of vitriolic vapours. By Monday, the budget was unravelling faster than you can say fiasco and George Osborne had gone into hiding. Mr Cameron had to face the Commons that afternoon. This should have been a horrible experience for him.

Yet still he was in luck. Good fortune smiled on him in the bearded guise of his main inquisitor. His prayers for relief had been granted by St Jeremy, the patron saint of prime ministers in peril. Presented with a priceless opportunity to skewer the prime minister and take apart his claims to lead a one-nation government, Mr Corbyn decided the most effective approach was not to mention the self-defenestration of IDS and his excoriating attack on the cabinet which he had just left.

The Labour leader did not, as some have had it, kick the ball over the bar. He didn’t even try to connect his foot with the ball. I am still trying to fathom why not. Had no one told him that a significant element of the job description of leader of the opposition is to, well, to oppose? Was he too preoccupied drawing up lists of suspected traitors among Labour MPs to prepare for this important engagement at the dispatch box? Was he too busy tending to his allotment and nurturing his marrows to have watched any news? Maybe I am over-thinking this. Maybe he is just hopeless.

If that performance had Labour people looking on in stunned disbelief, there was worse to come two days later at prime minister’s questions. By then, Mr Corbyn had managed to find out that a member of the cabinet had resigned. But it was too late. Bringing it up 4 8 hours on only served to remind everyone that he had failed to stick the ball in the net two days earlier. On top of which, someone on his team carelessly lost a list which divided Labour MPs into five categories of loyalty and opposition to the great helmsman of Islington. The list fell ino enemy hands. So we all now know that his chief whip is designated “hostile” and so is Labour’s candidate to be mayor of London. “Core group negative” includes Ed Miliband and Alan Johnson, the leader of the Labour In campaign.

The Tory leader used the exposure of the list to crush the man opposite. “I thought I had problems,” he jeered after saying they could put him down as “core support” for Mr Corbyn remaining as Labour leader. A half hour that ought to have been torture for the prime minister turned into a humiliation for his opponent.

The most devoted of Mr Corbyn’s followers will say that this mockery shows that the Tories are frightened of the Labour leader. Let me try to break this as gently as I can. The Tories really, really are not scared of Mr Corbyn. Most Tories are more likely to lose sleep worrying about whether they put out the cat than they are about the Labour leader. For reasons I will describe in a moment, the Conservatives would be better served, and so would the country, if they were a bit more frightened of Labour. The other thing people will say is that parliamentary knockabout excites only people who live in the “Westminster bubble”. No one “in the real world” cares about this meaningless theatre.

I agree that it is theatre, but it is far from meaningless. Parliament still matters for holding the prime minister to account. It matters more under this prime minister because he rarely deigns to grant substantial interviews with heavyweight media interrogators and only holds news conferences when he has absolutely no choice. The prospect of being tested by the leader of the opposition should, at the very least, make a prime minister nervous. It should keep him on his toes. Since he started facing Mr Corbyn, Mr Cameron clearly finds PMQs so effortless that the ease with which he cruises through them must embarrass even him. It also matters because how the party leaders perform in the Commons influences how the media rate and report on them and that plays its role in shaping public perceptions. It also has a significant impact on the morale of their parliamentary troops. At the end of that PMQs, you didn’t need any lists to tell you what Labour MPs thought about Jeremy Corbyn. It was written on their funereal faces. An encounter which should have united them in exploiting the government’s divisions and disarray turned into an occasion in which the Tories roared on their man as he ridiculed the Labour leader.

My first column of this year remarked that it is highly unusual for both major parties to be doing the splits at the same time. As this year grows older, things are getting even stranger. We now have a feedback loop in which the divisions in the Labour party feed those among the Tories and vice versa. Whatever their manifold and manifest differences, one thought unites nearly all Conservatives – that the next election is as good as won for them. This is encouraging Tories to think that they can behave however they like without fear of punishment at the ballot box.

The empty space where an effective opposition ought to be is an incitement for the government to be complacent, cocky and slapdash. That arrogance has consequences, as we saw with the budget. At the same time, absent an opposition that they fear, Tory discipline is breaking down. The prime minister’s internal opponents are emboldened to be more aggressive in their rebellions against the Tory leader. The language exchanged about Europe becomes more poisonous. It lessens the chances of putting the Conservative party back together again on the other side of the referendum.

In normal times, we’d expect this to have a positive effect on the opposition. The spectacle of Tories tearing into each other like a feral bunch of ferrets ought to be uniting Labour in a conviction that the next election is winnable for them. In these extraordinary times, Tory division has the opposite effect. It is not bringing Labour together; it is amplifying Labour’s internal turbulence. Labour MPs see a Tory party which is bitterly split and despair that their own leadership seems utterly incapable of profiting from the opportunity that it ought to present to them.

A few Labour MPs have broken cover and openly called for Jeremy Corbyn to resign. Many more talk privately of an attempt to oust him once the EU referendum is over. Whether this will come to anything, we will see, but I would caution Mr Corbyn not to place too much reliance on that list as a guide to the mood in his parliamentary party. It has Labour MPs down as neutral or friendly who are, to my knowledge, hostiles.

While talk of toppling their leader grows on the Labour side, regicide is also on the minds of a significant number of Tory MPs. Among them, there is much chatter that David Cameron will face a leadership challenge after the referendum, whether he wins it or not. It is possible that, before year’s end, there will have been attempts from within both the major parties to oust their leaders. What would once have seemed wildly improbable is now quite easily conceivable. The terra becomes yet more incognita.