Summer has surrendered to autumn, and in Westminster the wind has changed. When parliament broke for the recess, Ed Miliband was teetering off balance, buffeted by gusts of derision. Labour MPs looked sallow, in expectation of defeat. Their leader felt obliged to make a speech acknowledging how poorly he met most expectations of a dashing prime minister “from central casting”. He asked the nation to consider some other criterion when choosing a leader. When politicians plead for a second audition it is usually safe to assume they aren’t getting the part.

But most political assumptions these days are unsafe. Already it is David Cameron’s turn to look hapless. He has not thrived in a summer of overseas emergencies, despite confidence in Downing Street that statesmanlike composure is the prime minister’s most saleable quality.

Complacency on that front was exposed in the hollow bombast of Cameron’s response to the news that hundreds of British-born Muslim youths are taking jihadi gap years in Middle Eastern war zones. He ramped up the rhetoric of once-in-a-generation existential conflict, but then appeared in parliament dressed in caveats. There would be no “kneejerk” response; new powers would be the result of cross-party consultation; with a few tweaks, existing laws sufficed for now. No 10 craved the pleasure of a hard-hitting headline without the pain of grinding out a proper strategy.

It is the sort of trick that leaders get away with when they are new and rich with the benefit of the doubt, which Cameron is not. His act is looking jaded. Tory MPs, whose loyalty to the current leader is a jelly that never properly set, are wobbling all over the place. Boris Johnson distracts them with his fidgety ambition to be leader instead. Holders of marginal seats are finding their cash-strapped constituents unwilling to recognise the blissful economic dawn that is meant to herald a Tory general election victory. Many are totting up the number of ex-Liberal Democrats on their patch who are now firm Labour supporters and realising their majorities will be overcome, especially if hundreds of Tories flock to Ukip.

Downing Street has been counting on a mass repatriation of errant Farage-fanciers, who are supposed to take fright at the prospect a Labour government and come “home” to the Tories. The defection of Douglas Carswell, until last week the Conservative MP for Clacton, in Essex, tells a different story. If, as looks likely, Carswell wins a byelection on 9 October, Farage will have a neat rebuttal to Cameron’s warning that voting Ukip lets Labour in by the back door. Look, he will cry, how voting Ukip lets Ukip in by the front door.

Yet Carswell’s action does make it more likely that Miliband will be prime minister. It accelerates a civil war on the political right that most Tories knew was coming but which they hoped to defer until after Cameron had been reinstalled in No 10. If those hostilities are brought forward, voters will conclude that the best place for a party to tear itself apart is in opposition, and consign the Tories to that fate.

And so, without much movement in the opinion polls, Westminster’s mood has shifted. The herd instinct of the political trading floor is buying Milibands and selling Camerons, although not yet in great volumes. The wind could quickly change back.

While most of the Tory suffering is self-inflicted, it would be churlish to deny Labour credit for collective holding of nerve. There was a simple strategy for the summer: campaign hard and, as one senior Miliband aide put it, “no more fuck-ups”. By this was meant an end to the run of presentational accidents that made the leader’s team look amateur: the Liverpool-baiting photo of Miliband brandishing the Sun newspaper; his slovenly public wrestle with a bacon sandwich; failure to get a personalised message on the wreath laid at first world war commemorations. The plan worked, to the extent that Labour looked organised enough for the government’s floundering incompetence to shine without distraction.

That is only a partial comfort. Labour candidates in winnable seats do the same electoral sums that make Tory incumbents gloomy, but they worry that the advantage is going unpressed. Their canvassing unearths voters who reject Cameron for exactly the reason Miliband says they should: suspicion that economic recovery is bypassing all but the wealthy few. But that feeling is not being converted into appetite for a Labour government. A litany of policies has yet to crystallise into a clear sense of what would really change.

Luckily for Miliband, the Tories are doing their best to confound anyone trying to understand what would happen in the event of another Cameron term. Many of them seem to think Cameron himself would not be part of the package for long. Some are plainly determined to effect ideological congress with Ukip. Others wish their leader would resist that tendency. Few believe he will. Cameron leads the small section of his party that has jobs in government, while the rest have already made the mental leap away from the coalition and busy themselves thinking about what and who comes next.

Neither of the two parties that have dominated English politics for a century is facing the electorate with a clear sense of identity or purpose. The contest is close, but not in the form of a traditional race that advances towards a finish line. Miliband is becalmed while the Tories are rudderless. Neither is really moving. It is only a capricious Westminster wind that seems to carry first one and then the other closer to power.

Twitter: @RafaelBehr