The coup is not over; it has only just begun. This is how we get rid of sitting prime ministers in this country: a series of blows, each more lethal than the last. Margaret Thatcher had Nigel Lawson's resignation a year before she fell, Sir Anthony Meyer's stalking-horse run, Geoffrey Howe's resignation and speech, and finally Michael Heseltine's challenge. Tony Blair, under pressure from Gordon Brown, conceded that he would allow "ample" time for a "stable and orderly" transition, faced demands from MPs for a "timetable", carried out a defensive reshuffle after disastrous local elections in 2006, and finally, after his failure to condemn the Israeli invasion of Lebanon provoked the September coup, was forced to hand in his 12 months notice.

Gordon Brown faced the first, tentative rebellion last summer, when Siobhain McDonagh resigned as a government whip, David Cairns resigned as minister for Scotland when asked to condemn her, and David Miliband spoke ambiguously about renewal. Last week's was an altogether more serious enterprise, even if, as it turned out, it was only a dress rehearsal. We are now set on a trajectory towards Brown's departure before the general election.

Let us "step back and understand what has been happening", as Brown invited David Cameron to do at Prime Minister's Questions on Wednesday. There is one big factor that continues to keep Brown in No 10, and that is that most Labour MPs do not want an early general election, which is the unavoidable consequence of making the change.

That is why last week's convulsion in the Cabinet failed to unseat him, and it is why the aftershock that will run through the Parliamentary Labour Party as MPs return to Westminster tomorrow will not knock him over either. It doesn't matter how bad the European Parliament election results are tonight: they can be safely dismissed as a diversion akin to the Eurovision Song Contest, a competition between groups dressed in silly costumes.

But it is also why they will get Brown in the end. When he faces his next convulsion, probably in the autumn, the timetable will look very different. Alan Johnson will be able to come in and promise an early general election – in the spring of next year; that is, roughly, when there has to be an election anyway and nobody has anything to lose. Then, the only calculation that matters is, as James Purnell put it in his resignation letter, whether Brown's "continued leadership makes a Conservative victory more not less likely", or, as Stephen Byers asked more bluntly: "Is Gordon Brown a winner or a loser?" We know what most Labour MPs privately think is the answer to that.

There are secondary factors that saved Brown last week. One, of course, is his resilience – both psychological and organisational. He is not one of nature's folders of tents. And he knows how to neutralise threats. Another is that his opponents are rebels without a cause. What is it that Alan Johnson's supporters can say is the reason for replacing Gordon Brown as Prime Minister? That Labour would lose fewer seats under his leadership than under Brown's? That would be true, but they need a reason that is framed in terms of the national interest. Purnell tried, in his resignation letter: "This moment calls for stronger regulation, an active state, better public services, an open democracy." Brown parried, in his reshuffle news conference, setting out the "three legs" of his strategy, including public services tailored to the individual, constitutional reform and some other stuff, but people had already lost interest.

When the moment comes, later this year, a public reason for Brown's removal will be invented, or provided by events.

So why did Purnell strike when he did? It was, he told friends, "just a matter of telling the truth". But that truth – that Brown cannot beat Cameron – has been known to Blairites, and indeed to Tony Blair, since before Brown took over. This is where we need to depart from the model of politics that has informed much of the reporting of last week's events: of plots, conspiracies, networks and factions. Those are of course important parts of politics – the politics of calculation. But there is another aspect of politics, that of ego, pride, emotion and selfishness, which was a better guide to much of what happened last week. I was told that Purnell simply could not face going in front of the microphones on Friday to say why Brown was the right person to lead the party into the next general election.

Certainly, it was the politics of emotion that drove the resignations of Hazel Blears and Caroline Flint – and possibly of John Hutton, too. Blears was furious at Brown's condemnation of her property dealings when he defended similar transactions made by other cabinet ministers.

Purnell will not have calculated precisely. He may have thought David Miliband would join him, when he phoned to tell him he was resigning a couple of hours before going public. He may not have realised that his going sealed Ed Balls's fate. There were signs on Thursday before Purnell's bombshell that Brown was getting cold feet about sending Alistair Darling to the back benches. Darling is popular among Labour MPs in a way that Balls is not, and by refusing a move to another job he raised the stakes for Brown. But Purnell's challenge ensured that Balls's elevation to the Treasury could not happen.

Thus he has weakened Brown. Only the other day David Cameron called for a "radical redistribution of power"; this week he got it. A shift of power from No 10 to the great offices of state: the Chancellor, the Foreign Secretary (who also would have resigned if Brown tried to move him), the Home Secretary (and heir-even-more-apparent) – and of course the First Secretary and Grand Panjandrum, Lord Mandelson.

That is temporary, however. What Purnell has really done is make it more certain that Brown will go before the election. The point about a dress rehearsal is important psychologically. One of the barriers to a successful coup is that people cannot visualise it. Last week, we saw how it might happen. We saw more of Alan Johnson and it did not seem ridiculous that he might be prime minister.

Gordon Brown has survived, only to be more certain of going next time.