It was something of a shock to learn yesterday that I may be distantly responsible for Iain Duncan Smith’s resignation as work and pensions secretary. According to the Sunday Times he “has never forgiven Osborne” for the disclosure in my 2013 book on the coalition, In It Together, that the chancellor believed that he was “just not clever enough” for the post he held until Friday.

So it’s all inadvertently my fault, is it? I’m the Forrest Gump of the 2016 European Union referendum, the accidental protagonist. Well, maybe. Politics is indeed like a box of chocolates: you never know what you’re going to get. But after a weekend of investigation and deliberation, I am no closer to understanding why Duncan Smith quit.

Tories in civil war as Duncan Smith attacks austerity programme Read more

It is odd, for starters, that on the day he resigned he also sent a “Dear colleague …” letter to MPs defending the very reform of benefits for disabled people to which he supposedly objected so vociferously. Odder still that the contentious blueprint had just been postponed – precisely as he had wanted. This brings to mind Finkelstein’s law of the Tory right, coined by Daniel Finkelstein, the Times columnist and former adviser to John Major and William Hague: namely, that many of those on what has become the pro-Brexit right will not take “yes” for an answer.

Reading Duncan Smith’s resignation letter, one has to ask how well the quiet man actually understands the government of which he has been a senior member for six years. Its central project – like it or not – has been a programme of deficit reduction enacted in the name of economic stability.

In 2005, David Cameron and George Osborne inherited a party that had been dragged by Michael Howard from the abyss into which it fell during Duncan Smith’s spell as leader, and embarked upon a programme of brand “detoxification”.

After the financial crash, they took the risky step of warning that an “age of austerity” was at hand. These were big strategic moves, intended to steer the party back towards government. “When you hear Iain moaning about the things we had to do to get back economic credibility and into power,” says one Cameron loyalist, “it’s tempting to say: ‘No thanks to you, mate.’”

How quickly Tories forget what wins elections. If there was a single reason why the party secured an unexpected majority last year, it was because of the public’s perception that Dave and George would run the economy more reliably than the two Eds. In his resignation letter, Duncan Smith generously allows that “difficult cuts have been necessary”, but goes on to say that “there has been too much emphasis on money-saving exercises”. In other words, “fiscal nimbyism”: cut away, George, but not in my back yard. This isn’t outraged principle. It’s scalded proprietorship.

In a broader, more numinous sense, the countdown to 23 June is the context that is framing all Westminster politics

So, is this really all about Brexit and the referendum rather than welfare? It is not as though Duncan Smith’s departure will free him to speak his mind on the matter of Britain’s place in the EU: thanks to the suspension of collective responsibility during the referendum campaign, he already enjoyed that right. But in a broader, more numinous sense, the countdown to 23 June is indeed the context that is framing and defining all Westminster politics.

In practice, the prime minister is once again at the helm of a coalition, more fractious and riven with ancestral passions than its Conservative-Liberal Democrat predecessor. There is the Cameron party, committed to Britain’s continued membership of the EU, in no hurry to see the incumbent leave No 10, and hopeful (in most cases) that Osborne will succeed him. And then there is the Brexit party, loose-knit but united by the wish to see Britain escape the EU and (in most cases) for the Cameron-Osborne duopoly to come to an end. With a few exceptions, the Brexit party would like to see Boris Johnson installed in No 10 as soon as possible, even if they are not sure why.

The Labour regime of 1997-2010 was ruinously scarred by the feud between Tony Blair and Gordon Brown. But the Cameron government has suffered from precisely the opposite problem – the perception that the prime minister and chancellor get on too well, that the whole thing is an impermeable stitch-up and that only those who are trusted by the duumvirate get anywhere. There is a longing among many backbenchers to see this seal broken, and power spread more widely.

Duncan Smith’s letter has been correctly interpreted as an attack on the chancellor and his leadership ambitions. But it is also an attempt to embarrass Cameron and to portray him as a fading force.

“I believe the cuts would have been even fairer to younger families and people of working age,” Duncan Smith writes, “if we had been willing to reduce some of the benefits given to better-off pensioners.” This is directly aimed at the prime minister who has consistently ruled out any cuts to pensioners’ entitlements (as it happens, Osborne is more open to suggestion on this matter).

Tory civil war sparked by resignation – in quotes Read more

Duncan Smith’s point is that Cameron’s implacable protection of the elderly severely limited the scope for savings in his departmental budget. Unambiguously, the letter is a critique of the whole Cameron era, not just one of its most senior figures.

So what? Why the resentment of a lone former cabinet minister should be seen as lethal to either Cameron or Osborne eludes me. In spite of what you may have heard or read, his letter is no match for Geoffrey Howe’s resignation speech in 1990, the rhetorical attack that triggered Margaret Thatcher’s downfall. It is treachery with a confused expression.

So Cameron should not be alarmed by Duncan Smith’s fit of outrage. But he must take very seriously indeed those in his party who are proposing a leadership contest after the referendum, whether the outcome is to stay or go.

Don’t forget: the Tory rule book does not permit a leadership challenge. Instead, the incumbent faces a vote of no confidence (as Duncan Smith did in 2003) if 15% of MPs – 50, at present – seek such a ballot. If the incumbent loses, a contest is triggered in which the sacked leader may not stand.

It is beyond extraordinary that, less than a year after it won a historic election victory, led by a prime minister who has already announced his departure, the Conservative party is playing with the gelignite of a wholly unnecessary leadership race. Indeed, it positively craves such a contest, even if Cameron prevails in the referendum. This is patently madness, but it is a madness towards which the Tory tribe is lurching, encouraged by Duncan Smith and by a yearning for Boris Johnson.

Cameron’s instinct has always been to smooth over difference, restore harmony, unite factions. But this is different and he must be prepared this time to draw blood, destroy his foes and be ruthless.

The Brexit party is really pushing its luck. The question is: how hard and how soon is the prime minister ready to fight back?