It would take Clinton being replaced as the nominee*, because she is the only reason that Trump has as much of a prayer as he does. Trump vs anyone else would have been case closed by this point.
* assuming she isn't replaced with Bernie Sanders
Actually, this is, probably, almost the best imaginable way of improving Trump's chances. Replacing Clinton, assuming this is not done because she is asssassinated or something similar, means that the Democratic party is going to be severely divided for an extended period of time. Furthermore, Clinton political campaign is well-oiled and ready to go: anybody else would be struggling to improvise. Finally, whom would she be replaced with? The only realistic possibilities this late in the game are Biden or Kaine. Biden is only everybody's favorite grandpa because he is
not running. He is, actually, gaffe-prone, and would not be hard to demonize. Kaine, being Clinton's pick, would have to answer for whatever sins that get Clinton replaced.
