"It's taken many people time to understand that things really are as bad as they are in the UK," said Lotta Nymann-Lindegren, a former Finnish diplomat who focused on Brexit issues and now works at the Miltton consultancy. "It's been a real eye-opener that an issue like this can cause such domestic political chaos."
Although May could still pull off an upset victory, discussions in both London and Brussels revolve around the expected margin of her defeat on Wednesday NZT, some time after 8am. Scores of Conservative MPs have already declared their plans to rebel against May.
"I believe the Government is going to lose this vote, I hope - I'm afraid to say - the Government loses the vote. And then either this prime minister or, if she will not do it, another prime minister must take it back to the EU and change it," said Zac Goldsmith, a Conservative MP.
The deal reflects a delicate balance on Ireland, where Dublin has sought to maintain an open border with Northern Ireland to avoid reigniting the violent conflict there. London, in turn, has pressed to avoid any border between Northern Ireland and the rest of the United Kingdom. If negotiators fail to craft a different deal during a 21-month transition period, Britain would be locked into a customs union with the European Union, subject to many EU regulations and unable to strike most trade deals with other countries.
The compromise is painful, but May and her allies say it is necessary.
"This deal is the best deal to exit the EU that is available or that is going to be available. The idea that there's an option of renegotiating at the 11th hour is simply a delusion," British Chancellor of the Exchequer Phillip Hammond told Parliament last week.
If May loses, she could go back to Brussels, cap in hand, and ask for further concessions, as early as Friday at a previously scheduled summit. She would hope that any tweaks to the deal, plus perhaps panic from the markets, would be enough to persuade MPs to support it at a second vote.
If that fails, there could be a leadership contest, a general election, a new government, even a second referendum. There is also the possibility - as May likes to point out to the hard Brexiteers - of no Brexit whatsoever.
In Brussels, officials say they are willing to keep discussing the deal - just so long as nothing of substance changes.
They could offer nonbinding declarations to make clearer that the remaining EU members do not want to lock Britain into an economic marriage against its will. They could tweak the part of the Brexit deal that lists the aspirations for their future partnership, which does not have the force of law. If talks seem to be on track, they could nudge the Brexit deadline from March to late June, when a British-free new European Parliament will be seated.
"Usually there are some - I can joke - tricks," the frank-talking Lithuanian President Dalia Grybauskaite said last month of the way the European Union finds consensus among its many members. "We promise to promise."
Advocates of a hard Brexit claim that they still have leverage in Brussels because the chaos of a deal-free British divorce would also snarl European economies.
EU negotiators say the British are badly deluded and that their own business leaders actually fear a no-deal Brexit less than concessions that could give British businesses advantages in the vast EU market without the obligations of EU regulations and taxes.
Both sides are now drawing up emergency plans about how to minimise chaos if the worst happens. On the EU side, preparations have accelerated in recent weeks, as the depth of the British chaos becomes clearer, according to diplomats involved in the discussions. At the weekend, the British Government warned of border disruptions for up to six months if the United Kingdom crashes out of the EU.
Within a matter of days, the EU and Britain could impose measures to allow planes to keep flying, medicine and food to continue flowing into the United Kingdom, and British citizens living in Europe to remain there. But any plans will be temporary, analysts say, leaving deep uncertainty.
