I have been trying speculatively to figure out what sequence of events might make things really blow up for Trump. I do not think crowds or crowd estimates or negative signs or the like will do it, although I think all are useful. But there is one sequence of events that could, I am guessing, really pay off. If Trump were to try to do something that a court ruled illegal [such as the current ban against immigration from a number of countries], if Trump ordered Sessions as Attorney General to challenge the ruling at an appellate level, if he were then ruled against all the way to the Supreme Court [as I think would be likely if it was an open and shut case], I could really imagine Trump then declaring that as President he is above the law, which would create a full blown constitutional crisis.
Just a thought.