Some big questions will be asked this weekend – about how Labour fell so far so fast, about when Keir Starmer goes and who takes his place – but at least one big thing will be clear: never entrust your country to people who keep insisting they’re grown up.

Think back to 2024 and the birth of Starmer’s government. “The adults are back in the room,” exulted Darren Jones as Labour went marching into Downing Street. Having chopped the party’s largest pledges into little pieces (Goodbye, Green New Deal! Farewell, securonomics!), the single greatest qualification Starmer, Jones and co had for office was not policy, but vibes. After a decade of blue-on-blue fighting and a string of gap-year prime ministers, all the reds had to be was serious, sensible, businesslike. Labour would own the mien of production.

“Stability is change,” said Starmer, a phrase straight out of Chauncey Gardiner that still wowed commentators. Andrew Marr summed up the collective delight: “For the first time in many of our lives, actually Britain looks like a little haven of peace and stability.”

Less than two years later, Starmer’s people freely accept that Thursday’s elections will be an epic disaster. If Labour MPs follow through on threats to force out their leader, then in the decade since Theresa May took office the UK will have discarded more prime ministers than Italy. Britain will become a cautionary tale of the perils of leaving grownups in charge.

In Westminster, “adult” is a conman’s compliment. It sounds a judgment of character, when it is really a definition of ideology. One proves political maturity by not banging on about injustice, by not troubling too much the rich and powerful. To try it out, simply stand in front of a mirror and slowly declare yourself to be “pro-business and pro-worker”. If you can do that without flinching at the obvious contradiction, then congratulations! You too can be prime minister.

“Grownup politics” is praise wrapped around a sneer at those on the outside. Jeremy Corbyn will always be considered politically juvenile, even though he is in his 70s. Peter Mandelson, on the other hand, was the very definition of an SW1 “adult” – and we all know how well that went.

The most striking aspect of such phrases is how crucial they prove just as political authority is breaking down. When the people in charge can no longer make arguments for their positions, they shut down the argument entirely by pleading for “grownups”.

During Europe’s existential debt crisis, the International Monetary Fund’s then head, Christine Lagarde, called for a dialogue with “adults in the room” rather than a pesky Greek such as Yanis Varoufakis, who wouldn’t play the game, but instead kept pointing out the lies underpinning the European bailout. When in 2017 Donald Trump moved into the White House, military men and corporate bosses such as James Mattis and Rex Tillerson who joined his administration were lauded in Washington as “adult”. Roughly translated, that meant: these guys will keep him under control.

In Britain, battered by a historic banking crash, Brexit and an awful pandemic, there remains every reason to rethink the relationship between state, market and public. Indeed, there used to be a Labour leader who promised just such a thing: his name was Keir Starmer and he talked about the need for a new 1945. But if that guy ever meant it, he long ago got Zenda-ed out of Westminster. In their bid for power, Labour frontbenchers instead talked more about how adult they were. And so after 14 years of disastrous rule, the UK ended up with a Labour prime minister mouthing such hapless, deflated slogans as: “We are now into phase two of the government, which is where we focus on delivery, delivery, delivery.”

You won’t be able to move this weekend for commentators gravely intoning that Starmer lacks a vision for government. What they won’t say is that he eschewed any vision to get into government. His very lack of politics is how he became politically successful. It was the essential precondition for him to be judged by the rightwing papers and others as no threat – as, in other words, a “grownup”.

In this setup, the people in power are adults, while the voters are children who must always be told why they can’t have what they want; why there is no magic money tree to give that nurse a pay rise, or why fiscal rules are more important than their kids’ futures. The breakdown of the two-party system, which was evident even in the 2024 landslide and will be the key talking point of these local elections, is a clear sign that voters are no longer willing to consent to these rules.

“Reform, the Greens and the nationalists are all eating into our vote because they can name something people feel in their daily lives – the system is broken – and point to who broke it,” Labour ministers now tell journalists. “We don’t even have that.”

The problem the governing party has is that its entire programme – the manifesto, the missions, the milestones – was predicated on the system working. That a tidal wave of foreign money would roll across these shores; that the economy would be the strongest in the G7; that the machinery of the state shown up as so inadequate during Brexit and Covid would purr like a Rolls-Royce. None of that happened, which leaves the adults of Westminster looking for someone to blame. This weekend the scapegoat will be Starmer.

Whoever replaces him as prime minister cannot promise business as usual. However, they will be very limited in what they can promise because Labour’s mandate is based on a very narrow manifesto. Whatever big ideas are floated by allies of Andy Burnham, say, they will run into the stony ground that they have no electoral backing. The first challenge Nigel Farage and Kemi Badenoch will throw at Burnham is that he wasn’t even elected an MP in July 2024 – and they will have a point.

This is where the pose of “adult politics” lands a government: premises and promises binned, alibis exhausted, voters mutinous. And crucially, the desire to present opponents with as small a target as possible now leaves ministers with almost no wriggle room. They can throw the boss out of No 10, but they can’t so easily free themselves of the ideology that put them in power.

The political stakes in Britain remain big. It’s the grownups who got small.

• Aditya Chakrabortty is a Guardian columnist

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