Boris could always implode. He looked decidedly shifty in the two public events this week when his minders allowed him out, more like a hostage, or perhaps someone in a police interview trying to avoid saying the wrong thing.
His unambiguous statement in an interview today that the only time he ever tried cocaine was on one occasion when he was 19 could prove dangerous. It doesn't sound too plausible to me, and if it isn't and someone grasses him up it could cause pretty serious problems for him.
It looks pretty certain he'll be one of the two finalists, and on the face of it that should ensure he's home and dry. Unless he blows it.
The thing a lot of the MPs backing him are relying on is his dishonesty and lack of principle. Just because he promises something disastrous to get elected doesn't mean he'll stick to his words.