Imagine a moment you're Boris Johnson (hard, but try).
You're immoral, disloyal & dont give a damm about anything except being Prime Minister. Now do you vote against the deal even if some ERGers vote for it?
Well theres a cynical reading of Boris strategy that explains why..
Now Boris who famously wrote letters both ways before #Brexit has if you think about no real ideologcal committment to brexit unlike much of ERG.
So cynically he will be one of the last opponents to back May's deal because if UK does remain he can then blame May...but if..
But if theres no deal then Boris will have polished his hardliner credentials. Voting for May's deal compromises him with no upside he doesnt get by sticking to no deal.
Boris likely isnt only one. Many supposed #Brexit hardliners will thus have an interest to opposing May's deal even if this means remain - if they put their leadership chances before the country.
Thus ironically the hardest brexiters cld be remainers closest "allies".