One of the big reasons Brexiter Tories don't want the UK in the Northern Ireland backstop is that they fear the UK out of the EU but in the backstop would continue to be a rule taker from Brussels till we all die.
And one of the reasons Labour Brexiters have struggled to support the Prime Minister's version of Brexit is that they are not confident a future British government would continue to adopt the high employment and environmental protections that the EU has championed.
To use the emotive language of Boris Johnson, Tory Brexiters want no Brussels vassalage (or the absolute bare minimum) and Labour Brexiters want some vassalage.
So perhaps the best measure of the PM's desperation to win the vote on her deal is that she is trying to appease both sides: she is supporting the Swire amendment to her "meaningful vote" motion on her Brexit plan which would supposedly help to get the UK out of the backstop after a year, and considering support for the Mann amendment that would see the UK following EU employment and environmental standards in perpetuity (and even adopting revised EU standards after Brexit).
For the prime minister, this would be just a sensible compromise to avoid what she sees as a damaging no-deal Brexit - what Tony Blair used to call a "third way".
For her critics it would represent the unsustainable, irreconcilable contradictions in her Brexit proposal, and is proof why a majority of MPs will never support it.
Apart from anything else, neither amendment - even if passed - would have legally binding force, for the simple reason the backstop is part of an international treaty that a parliamentary vote cannot over-ride whereas the future of our labour and environmental standards would be set by negotiations with the EU that cannot happen till after Brexit.
All of this is a piece with the prime minister's blancmange of a Brexit, which has wobbled, changed shape and been squished by the varying political pressures in Westminster and Brussels.
But if she does lose the vote on Tuesday, as widely expected, that would be the moment Brexit can no longer be quite so gelatinous and malleable.
At that juncture she would have to decide whose side she is on, the Brexiter ultras or those in both her party and Labour who actively seek a bit of long-term vassalage.
But either choice risks the break up of her party.
That is why, perhaps, she may ultimately conclude the only way through is to announce a formal surrender and turn the decision of what kind of Brexit or no Brexit the UK ultimately adopts over to MPs (who could also decide that it's all too hard, and would then throw the decision back at us, in a referendum - gawd help us).