Something, at last, has changed. For nearly a year, Brexit has been characterised by a weird combination of turbulence and stagnation. The outline of an EU withdrawal agreement was agreed last December. The obstacle to its completion was apparent then, too. It was impossible for the UK to leave the European customs union while also retaining frictionless trade and without sabotaging the Good Friday agreement.

For as long as Theresa May insisted that was not the case, progress in negotiations was glacial. News from Brussels was rare. Cabinet feuds and backbench posturing filled the gap. There was noise but it was a cacophony of questions: could the prime minister complete a deal and, if she did, could she sell it as a success to her party and to parliament? What would become of her – and of the country – if she could not? Today begin the answers.

Ministers will decide whether to hitch themselves to the text that the prime minister has brought home. They will weigh the high historical stakes and factor in some lower calculation, starting with the parliamentary arithmetic. May needs the votes of 320 MPs to get a deal through the Commons. On the basis of conversations around Westminster in recent weeks – and there has been talk of little else – no one thinks she has the requisite tally – yet. And if there isn’t a majority for the deal, if it is destined to be a dud, there is less incentive for the cabinet to invest their political capital in her.

The view in Downing Street has always been that the fact of an approved text would transform the debate. Instead of speculating about what should happen, MPs would be forced to confront what will happen if, so close to the deadline, they reject the actual Brexit on offer. No 10 has been priming the spin pump in anticipation of this moment: the heroic 11th-hour clinch of a deal, in defiance of the naysayers, the one and only chance to honour the will of the people and move on. May is also betting that a hitherto disengaged portion of the public will tune back in for the deal, that they will not be too fussed about the details, be glad that the end of the odyssey is in sight and be appalled if parliament looks poised to wreck it. The great Brexit spirit god of JGOWI will be invoked: “Just get on with it!”

The problem with that pitch, as both hardline Brexiteers and unrepentant remainers point out, is that May’s deal will not really settle the matter at all. The withdrawal agreement, running to hundreds of pages, covers only the practical moment of separation. It is only a licence to proceed into transition, where questions of long-term alignment with the EU must then be finalised. The document prefiguring that part of the deal, the “political declaration” on a future relationship, is flimsier. Many of the biggest strategic and economic decisions will be kicked down the road. A lot depends on whether MPs see that imprecision as an opportunity or a threat. Some Brexiteers will see merit in bagging the simple fact of legal departure from the EU, with a view to dismantling any remnants of integration down the line.

Some pro-Europeans might have the same idea, but with a symmetrically opposing trajectory in mind: take May’s halfway-house Brexit, tick the box of satisfying the referendum mandate, and then start rebuilding bridges to the continent.

This is a question of temperament, and appetite for risk, as much as politics. The fate of May’s deal will be decided in a competition between two coalitions delineated neither by traditional party identities nor the more recent remain-leave schism. There will be a camp that swings behind May because there are just too many variables on the other side of her defeat. Each one of them is someone’s worst nightmare: exiting with no deal; a general election; Jeremy Corbyn as prime minister; a second referendum that cancels Brexit altogether.

On the other side are those who see at least one item on that menu that is just too desirable to pass up. They will not endorse a dismal compromise plan when their ideal outcome depends on killing it. No one sincerely believes that May has secured the best possible outcome. She had a duff hand, which she played badly. But in a stick-or-twist situation, when it is the nation’s prosperity and stability at stake, a lot depends on how MPs manage their fear of a bust.

During the past few weeks two contradictory propositions have been circulating in Westminster with roughly equal currency. One is that somehow a strain of pragmatism that is innate to British politics, coupled with raw terror of chaos, will drive May’s bargain into law. The other is that, with hardline Tories, the DUP and Labour all finding reasons to hate May’s plan, there simply aren’t the votes for it. Only one of those propositions can hold.

Overnight, Brexit has ceased to be a haze of slogans, ambitions, pledges and myths. It is now a piece of paper in the prime minister’s hand. There is no more “Brexit means Brexit”. There is a deal that either serves the national interest or betrays it. The detail will emerge in the days to come, but two things can be said with certainty. First, May’s route presents safer passage to the future than is envisaged in the wild fantasy of quitting the EU with no deal at all. Second, the terms that May has negotiated offer inferior status, market access, influence and power on the European continent and in the world than Britain has enjoyed as an EU member. Plenty of MPs know it, too; comfortably a majority. The question now is how many have the courage to say so.

• Rafael Behr is a Guardian columnist