Perhaps in your weaker moments you feel for Theresa May. She maintains an admirable composure when assailed by Nigel Farage, Boris Johnson and Arron Banks, privileged louts, who would long ago have been on antisocial behaviour orders if they had been born on council estates. She has a plan for Britain’s future: a humiliating plan that will throw people out of work and leave us at the mercy of laws we have no say in drafting. But when Labour and the Tory right have no plan, she can seem, in the irritating words of her admirers, “the only grownup in the room”. If you have twinges of sympathy, please crush them. The “grownup” cliche encapsulates everything that is wrong with her premiership. For if she is the grownup, the voters are the children who must be protected from reality so they can sleep easily in their beds.

The infantilising of the British stopped for a moment last week when the government belatedly managed a “there is no Father Christmas” moment. The Treasury said that Britain would be worse off under any form of Brexit.

The admission that May’s was the first administration in British history to deliberately make the country poorer was a small advance. For, ever since the referendum, May has failed in her duty to state the obvious.

So many on the right reveal their vanity and insecurity by claiming Churchill’s mantle. Say what you like about the old brute’s policies against the subjugated peoples of the empire, he had the honesty to tell the Commons in 1940 that he had nothing to “offer but blood, toil, tears and sweat”. By contrast, his supposed imitators promised us the impossible: a no-sweat Brexit, without one tear shed or drop of blood spilt. When May said she wanted to be “straight with people” and face up to the “hard facts” of Brexit, the only warning she could utter was the hopelessly inadequate banality that leaving the single market will mean the UK and EU enjoy less access to each other’s markets. No more straight or factual was her assertion, upheld until September 2017, that no deal was “better than a bad deal”.

May’s government was not opposed to the Brexit right. She did not come into office and lay out the hard choices it had ducked. For two years, she said next to nothing to contradict the slogans of Johnson, Farage and Banks. The dishonesty ought to temper your willingness to believe the rightwing voices who now insist that millions who voted to leave voted to stop EU interference or control immigration did not care about the economic consequences. They certainly worried about sovereignty and migration, but the men who led them on assured them they could have it all and the economy would look after itself.

Speaking of immigration, Sajid Javid, the home secretary, told parliament last week that he has delayed publishing the government’s plans on the future of the immigration system yet again. We’ve waited most of 2018 for them and may not see them before the Commons votes on May’s Brexit deal. Theories abound on why the government won’t be “straight with people”. The Home Office may want to hide the economic consequences of May’s tough line. Anand Menon and Jonathan Portes’s academic work estimates that May’s restrictions on migration alone will lead to a 1.4% to 1.8% fall in GDP.

Pollsters are noting, meanwhile, that mothers of young children are among the Leave voters most likely to switch to Remain as they see how the loss of European workers is harming the NHS and schools. Perhaps the government does not want the electorate to know the true cost of May’s crackdown.

In normal circumstances the opposition would expose the government’s evasions. But we have no opposition

Or perhaps there will be no crackdown worth mentioning. Yvette Cooper, John Woodcock and other centre-left politicians on the home affairs select committee believe the rumours that Javid is fighting May to ensure business has enough migrant workers once we leave. If he is, and if he wins, what will have been the point of the immigration “debate”? The government will just have replaced EU migrants with migrants from the rest of the world, trashed the economy and Britain’s standing in the world in the process, and removed the rights of the British to live, work and love where they choose in the EU for good measure.

A crisis of trust in politics is coming. May is making the same mistake as David Cameron. He spent years denigrating the EU and allying with its creepiest politicians and then believed he could pirouette on his Oxford brogues and instruct the public to vote Remain in 2016. May did nothing to combat the hard-right narrative that no deal was no big deal for 18 months and now believes she can warn of a catastrophe and persuade the public to force their representatives to support her – just like that.

In normal circumstances, the opposition would expose the government’s evasions. But we have no opposition. Conservative leaders do not tell the truth about the economic consequences of leaving the single market. Labour’s leaders don’t enlighten us either. Instead, they give us a lie as shameless as anything Johnson put on the side of a bus and pretend we can have a jobs-first Brexit. Equally, May is determined that freedom of movement must stop at all costs. As is the supposed internationalist Jeremy Corbyn. Neither the government nor the official opposition has begun to prepare the electorate for hard choices or found it in themselves to admit that hard choices exist. They have put maintaining their ramshackle coalitions above basic truth-telling.

Politicians play with fire at their and our peril. They have so lowered the bar of public life that one day a British Trump will skip over it with ease. If “the only grownups in the room” treat the voters like children for long enough, they will eventually throw a tantrum.

• Nick Cohen is an Observer columnist