It sounds like Labour will definitely vote against...and by the time the EU negotiators have had a go even more Tory support will peel off.
I'm tying to guess what will happen next - curious what others think of this:
...there wont be a vote on it because it will be dead in the water before it ever gets to that point, the inability to pass it will mean May has officially lost authority and will have to stand down. Or the vote will take place for the sake of it and it will get voted down and the effect will be the same.
An internal Tory election solves nothing, and will be done to the ticking of the Brexit Day clock.
A general election may likely take place before March 2019, but even if it doesnt, in terms of Brexit Day itself I think there are only two realistic options: crash out in WTO rules or some kind of process, which would have the backing of the majority of the commons, would stop Brexit happening completely. Cue a cultural civil war + constitutional crisis.
??
The only other option is that the negotiated Brexit bill passes the vote in the house of commons somehow (cant see how though). That would still do for May and see a Tory implosion I'd expect
No wonder Labour were all smiles at not winning the last election - dodged a bomb here, even if they get hit by the shrapnell