First off we don't know who the democrats will end up with as their nominee. Should Hillary get indicted and Bernie take the helm Trump will wipe him out. Polls showing Bernie beating Trump are meaningless as there has been very few negative ads run against Bernie to date. When people are educated as to what Socialism is and they start paying attention to a hunched over old man, Trump beats him in a landslide.
Assuming Hillary is not indicted but recommended for indictment by the FBI after she grabs the nomination she will be able to march on as an indictment is only an accusation which she can say is not justified. Naturally under these conditions Trump wins big.
Assuming that Hillary is not indicted and there is no recommendation for indictment it will be a much closer race. But, I will call Trump for a few reasons:
A. He will gain more democrat votes than any candidate since Ronald Reagan as Therajraj has already stated probably in the 20% range. This means he possibly wins in states that republicans have not won in years, PA, Michigan, Wisconsin, Virginia could all go to Trump. If you add the traditional red states and throw in Ohio which he will win and Florida which if he wins a few blue states he won't need and you have a Trump victory by a reasonable margin.
B. Hillary's negatives are currently at 61% Trump's are at 56%. Both incredibly high per historical levels for Presidential candidates. But nonetheless Trump has already lowered his incredibly high 67% negative rate by 11 pts. in only 30 days or so. As I have been saying people are fickle.
C. It has already been said on this thread that Trump has received more primary votes than any other republican in history. And he did it mostly running against a multitude of candidates. That momentum will continue whereas Hillary has no momentum and no excitement attached to her campaign. That could change but for some reason I doubt it that happens.
Finally, if Hillary is indicted before the convention and they toss her to the sidelines along with Bernie and parachute in someone like Joe Biden there will be hell to pay with the Bernie sanders followers along with many feminists who would prefer Warren. In that case I still see Trump winning.
So, I'm thinking Trump wins almost regardless. But the democrats could pull off some sort of genius move to beat him as he is easily beatable. However, I will stick to my long held prediction that Hillary Clinton will never become President. Not because I don't like her but because she is the single worst candidate in every category that the democrats have put up (if they do) in modern times.