Theresa May’s Irish backstop is Donald Trump’s wall. It is the howl of power thwarted by the implacable opposition of a democratic assembly. The prime minister today again sets her face against MPs who are now desperately searching for a parliamentary consensus on how to leave the EU.

She will plead with her Tory right wing – which is what it is – to support a supposedly “tweaked” deal on the Irish border under Brexit. For two years, experts have hunted the fantasy of “Brexit with an open border”, and found only a contradiction in terms. You cannot have a customs union and not a union. Yet May apparently believes she can convince her ever more ramshackle coalition to the contrary. This time she has hours rather than days to prove it.

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From the outset, May rightly sought a Brexit deal on which parliament’s majority parties could agree. It would be impossible to rule with authority with a third of her MPs against her, and only Jeremy Corbyn’s Labour in support. Last week, that tactic fell on its face. When invited to call Corbyn’s bluff and agree to remove “no deal” from the table, her nerve failed her. She could not bring herself to abandon party unity in favour of national interest. She decided on a reckless last throw.

Unless May is divorced from reality, she must know that the option being promoted by a growing number of senior MPs – from all parties – now makes most sense. As set out by Labour’s Yvette Cooper, the Tory MP Nicky Morgan and others, it postpones article 50, rules out no deal, sets aside past “red lines”, and sets up a brisk debate on the sort of Brexit the country really wants.

That debate should have taken place two years ago, and May is to blame for not instigating it. Such a debate is not, as the international trade secretary, Liam Fox, said on Sunday, anti-Brexit. Indeed it would be a useful concession to the pro-Brexit public if reversing the referendum were left off the agenda. It should embrace the proposal for a citizens’ assembly, backed by the Guardian and Gordon Brown, among others. It would be aimed at answering the question the referendum glaringly failed to answer: if Brexit, which Brexit?

For May to restrict this debate to a hardline minority within her party was inexcusable, and has not worked. There is emphatically no parliamentary majority for crashing out of the EU without a deal. Yet May is now risking just that outcome. Parliament has to form itself into a coherent collective – and stop her.

• Simon Jenkins is a Guardian columnist