The debates mean pretty much squat. One can lose the presidency via a poor performance but no one has won the Presidency via an exceptional performance. Why is that? Because they are not debates. They are joint, bilateral press conferences. The moderator will ask questions. Which is the problem. In a real debate the opponents ask each other questions. The moderator only intervenes if a participant violates the rules. They do not ask questions.
So instead of a real debate you get a joint press conference where the moderator, always a journalist, ask questions, and the politician responds, usually, with well rehearsed canned talking points.
There have only been two debates in my lifetime in which the Debates played a significant role. 1980 and 1988. In 1980 Carter, the technocrat, blew the so-called debate not on the technical merits of his arguments. He completely rolled Reagan up there. Carter offered specific answers to complex policy issues. Reagan offered glib platitudes. The difference was in tone. Carter came off as extremely pessimistic and Reagan came off as a bright and shiny optimist. Carters polls numbers dropped significantly after that but not as much as is often claimed. Their debate was rare in that it had an impact on the outcome but it is often exaggerated.
That's not the case in the 1988 so called Debate where the CNN Moderator asked Dukakis a completely inappropriate question about rape and murder and Dukakis panned the answer. He went from a substantial lead to losing the whole shebang.
Note to Clinton. If a moderator ask you what you would do if a criminal sodomized and murdered Bill...tell them you would shoot the mother fucker.
If Trump tries tonight what he did in the GOP primaries, he's sunk. The biggest problem Trump has to deal with is the perception that he lacks the temperament to be President. Trump will not only clearly need to elucidate clear policy points, which he has yet to do, more importantly he will had to do it with some gravitas. The perception about Trump that really hurts him with the part of the electorate he needs the most, college educated whites is that he's perceived as a bombastic, egotistical, salesman with a line of bullshit sales pitch that only a prole would fall for. He needs to make deep inroads there.
Clinton on the other hand needs to go on the attack. Her wait and let Trump shoot himself in the foot is only partially effective. He certainly has shot himself in the foot enough but she has to be more aggressive in attacking him on that.
I'll watch tonight and I'm sure there be lots of partisan claims of a great victory by both sides partisans but I'm going to watch it with the same attitude that I have with most so called Presidential debates. That it won't amount to squat, which has been the case with most of them.