The talk of the corridors of Parliament is that the PM is about to cave and accept a quartet of Brexiter amendments by the European Research Group to the Taxation [Cross-border Trade] bill (known as the customs bill).
The three important amendments would be:
make it unlawful for the PM to agree to separate customs arrangements for Northern Ireland than for the rest of the UK
prohibit forming a customs union with the EU for the whole UK without parliamentary approval via full legislation
and make it illegal to collect customs for the EU unless the EU collects customs for the UK
Now the first amendment is May's official policy, so presumably she can and will simply embrace it.
And the second should also not be a problem for her, in that May has said both that she opposes membership of a customs union and is not prepared to make any more concessions to the EU in Brexit talks.
But if she accepts that the UK would only be a collector of customs for the EU if the EU were to reciprocate for the UK, the centrepiece of her Chequers plan for the UK's future relationship with the EU, the so-called New Customs Partnership, would effectively be dead.
The tawdry negotiating reality, recognised by Whitehall, is that there is next-to-no chance of the rest of the EU sending money to the UK in respect of tariffs due to the UK on exports from non-EU countries that come to the UK via an EU country.
For the rest of the EU, it would simply be too much fuss and bother to collect customs due to Britain (and yes I know that isn't fair, given that we would be promising to do just that for them - but in this negotiation, they hold rather more cards than does the UK).
So if May were to cave to the ERG on the customs reciprocity arrangement she would be recognising arguably that her Chequers plan has died shortly after birth.
That would be a personal catastrophe for her, and would also raise grave doubts about whether any trade deal with the EU would be negotiable.
And there is another reason why she would probably be ill-advised to capitulate on all three ERG amendments.
The rebel Brexiters won't believe she has really abandoned her Chequers plan unless and until she explicitly says so.
They would take her concessions, even the one on customs reciprocity, and bank them. But they would see them through the prism of her continuing to buy time, to kick the can down road, rather than of being a Damascene conversion.
So if she doesn't stand and fight now, the ERG will keep coming for her - till she signs up for a Brexit that transfers more lawmaking powers to parliament or quits.
So if she isn't prepared to face down the ERG now, she would be their hostage. Which would then risk the Rebel Remainers coming for her.
If prime minister isn't prepared to stand her ground now, it is hard to see how she can remain PM for many more weeks.
As I said earlier the idea of the PM inviting a vote of confidence by Tory MPs in herself is not as mad as it may seem.
She has little personal authority to lose and some persuasive power to gain - if her MPs back her to stay in post, rather then triggering a leadership election. But it would be a dice roll.