It seems to boil down to May trying to drag the vote over the line by saying "I know you hate this deal but if you vote it down then Labour might get in and you'll hate that even worse". A truly inspiring battle cry which I'm sure you'll all agree could win over any of those who are undecided on which way to vote, it just needs a "Now cry God for Theresa and Saint George" on the end.
Sad thing is though, it might just work. I reckon that the hardcore Brexiteers are a busted flush now, they couldn't force through a vote of no confidence and they surely recognise they can't force through a harder Brexit. So they could be thinking that if they vote against May then they might endanger the whole leaving business. But they totally hate her deal cos it leaves us utterly servile to the EU, though arguably they could claim a "We left the EU so we won" victory of sorts, it's about whether that is more important to them than their professed principles.... hmmm.