It would be tempting to call this the moment of truth, had truth not been an early casualty of a Brexit saga that was mired in lies and deception from the very start. Even so the Brexit story, which has twisted and tormented this country for the last three and a half years, is at a moment of decision. Outside parliament, hundreds of thousands will gather to make one, possibly last, plea to stay in the European Union. Inside, MPs are due to vote on an agreement that, if it passes, will see us make the break in less than a fortnight – thereby ending British participation in a dream that has animated Europe ever since the final bombs fell in 1945.

Almost everything about this moment deserves either regret or condemnation. Forty months have passed since the referendum, but MPs will have little more than four hours to assess the new withdrawal agreement governing Britain’s departure from the EU. That’s barely time to read it, let alone debate and scrutinise it. To rush through a decision of such gravity is not the action of a country that is serious about its own future.

In return for their readiness to wave goodbye to Northern Ireland, the Tory ultras are getting a harder Brexit than May ever offered

That in itself is a good reason to support Oliver Letwin’s amendment, which would force the government to seek another EU extension and give everyone more time. (Letwin’s prime purpose is to head off a bit of Spartan chicanery, making it impossible for the hardcore Brexiters to get round the Benn Act by voting yes on Saturday, only to vote down the withdrawal agreement later, thereby triggering a no-deal exit on 31 October.)

What debate there is will be blind to the most crucial facts. The chancellor, Sajid Javid, has refused to provide an economic impact assessment of the deal, breezily insisting that any cost will be worth it because getting Brexit agreed will be “good for the fabric of our democracy”. The closest guide we have is an independent study, warning that the latest arrangements could reduce Britain’s per capita GDP by up to 7% over 10 years (compared with remaining), making this deal even more economically damaging than Theresa May’s.

And yet it is being hailed as a great prize, and Boris Johnson lauded as a master strategist. That is baloney. Once Johnson scrubbed out what had previously been a red line – no border in the Irish Sea, lest that weaken the union of Great Britain and Northern Ireland – then of course the EU could offer some new options. That doesn’t make Johnson a negotiating genius. It just means he was ready to – what’s the word? – surrender a principle that for the leader of the Conservative and Unionist party should have been sacrosanct. If you’re buying a car and suddenly agree to give up on having back seats, the price will go down. That doesn’t make you a tactical mastermind: it just means you gave up on something you previously insisted was vital.

News reports keep insisting that the backstop has gone, and Brussels is happy to allow that impression; but that too is false. It has, as one former Downing Street adviser puts it, become the “frontstop”, meaning that what was once a contingency is now a certainty. Northern Ireland will be, in effect, in the EU customs union from day one, even if it officially remains part of the UK customs area.

Credit to the negotiators for imaginatively finding that inch of space between de facto and de jure, and good for Northern Ireland for retaining at least some closeness to the enormous market on the UK’s doorstep, even as the rest of the UK needlessly distances itself from it. (Don’t blame remain-voting Scotland if it wants that too.) In a telling slip, the foreign secretary, Dominic Raab, told Northern Ireland to cheer up and realise it was getting a “cracking deal” that would allow it to keep “frictionless access to the single market” – which rather highlights why this is a terrible deal for everyone else.

An anti-Brexit protester outside the Houses of Parliament. Photograph: Xinhua/Rex Features

Still, there’s no dressing it up. Northern Ireland is being cut loose, by the same Boris Johnson who less than a year ago was telling the DUP’s annual conference that any customs or regulatory distinction between Britain and Northern Ireland would “damage the fabric of the union”, and: “No British Conservative government could or should sign up to any such arrangement.” How is this suddenly acceptable to today’s Tory party? Because, sighs a former minister, his colleagues “don’t give a shit about Northern Ireland and don’t care about the union”. Only Brexit matters now.

In return for their readiness to wave goodbye to Northern Ireland, the Tory ultras are getting a harder Brexit than May ever offered. The new withdrawal agreement is replete with changes that will put ever greater distance between Britain and our neighbours – measures that, as Labour’s Keir Starmer rightly points out, will “slash workers’ rights, environmental standards and consumer protections”. It beggars belief that any Labour MP worthy of the name could possibly vote for it.

The most wild-eyed Brexiters have spotted something else too. If they vote for this new agreement, they need only sit tight through the 14-month transition period and then, if no free trade agreement has been sealed by the end of 2020, they’ll get the no-deal crash-out of their dreams.

For that reason, and because there are pro-Brexit Labour MPs who reckon voting aye will bring only a mild slap on the wrist from a leader whose purposes it might suit to have Brexit out of the way (so long as no one blames him, that is), Johnson may just scrape home. If that happens, there will be many millions in this country who will feel nothing less than bereft.

Since 2016, remainers have often been rightly rebuked for failing to respect or even understand the sensibilities of the 17.4 million of their fellow citizens who voted to leave. But 16.1 million Britons voted to stay in the EU, and they have often been dismissed no less casually, whether rubbished as elitist “citizens of nowhere” or mysteriously excluded from “the people” whose will must be done. Yet they too are devoted citizens of this country, and, very soon, they may face a change that, to many, will feel like a grievous loss.

For three and a half years, they have put off that moment of pain, hoping that somehow, Brexit might be averted, that their fellow Britons would change their minds and change course. Sometime on Saturday that dream could be over. For all the procrastination since 2016, it will feel quite sudden: 31 October is very near.

They will contemplate a project whose importance to them – like so much in life – they didn’t fully appreciate until it was nearly gone. Yet in these last 40 months, often unheard amid all the noise about process, about meaningful votes and second referendums, they have come to value it very dearly. It is an ideal of cooperation across borders; of their country combining with its neighbours, rather than fighting against them, to face down shared threats, whether they be the climate crisis or the lethal recklessness of Donald Trump. In 2019, that idea seems more necessary than ever. Yet tomorrow it could all vanish.

So forgive them if they shout themselves hoarse at those demos and rallies, or if they watch the Parliament channel through their fingers as they wait anxiously for the vote. They – we – fear we are about to lose something very precious.

• Jonathan Freedland is a Guardian columnist