The ceiling is 538 if Donald Trump makes a comment like "You should take beauty tips from Rosie O'Donnell!" during the debates and Gary Johnson steals enough votes from him.
I doubt any disgusting misogynistic attacks are going to have much affect on his viability. This is a known quantity, and is already baked into the polling. What might cause him to tank is if Trump tells Clinton to her face "you should be locked up," and Hillary handles it perfectly. That's a weak volley and face plant, giving Clinton the perfect opportunity to slam the ball for a winner.
