There are several reasons that, barring some major gaffe on Trump's part, he should win the debate tonight with Hillary Clinton.
First, all you have to do to win this kind of debate is exceed expectations. As it stands now, the expectations for Trump's performance are lower than Hillary's, giving him a much lower bar to clear.
Second, Hillary faces a dilemma that Trump does not: She either plays it conservative, in which case she appears too boring and Establishment, or she plays it aggressive and tries to trash-talk Trump, in which case she will get the short end of the stick because no one can out trash-talk Trump.
Given these realities, Trump's strategy should be purely reactive: If Hillary remains docile, just stand there, make a few policy statements and look presidential, in which case he will have exceeded the expectations. If Hillary goes on the attack, then deploy the email scandal, the Bengazi scandal, the Goldman Sachs connection, and her husband's Bimbo scandal, in which case he will at least neutralized her attacks and possibly draw blood.
Thirdly, Each candidate comes with a constructive and destructive narrative and both will be trying to play to one and downplay the other. Hillary's positive narrative is that she is competent and tested; her negative one is the mirror image: she is an establishment figure who skirts ethical rules and gets away with legal transgressions that mere mortals would do time for. Trump's positive narrative is that he is an anti-establishment figure bent on shaking things up; his negative narrative is that he is impulsive and reckless with his words and a lightweight when it comes to the serious policy work it takes to be the leader of the Free World.
That's pretty much a draw. However, Hillary has one more narrative working against her, and it has to do with her health. Up until about a week ago, I thought her health really was a tempest in a teapot that no one really cared about. But I think that I thought that because I didn't care about it. As has become evident, a lot of people do care about it. The video of her collapsing as she got into a car made it an issue.
This debate is supposed to be 90 minutes without commercials. Do they even get a potty break? If not, Hillary has another dilemma: If she drinks water before the debate, then she's going to need one (remember the several times in the primary debates when she disappeared from the set?); if she does not drink water, she faces the problem of dehydration, in which case she faces the problem of passing out again (this was the reason given for collapsing in the now famous video).
Again, Trump doesn't face the same dilemma. If we can go by past experience, he can hold it--and he is more likely to make other people pass out than to do so himself. In fact this is the one thing that could completely change this election: If Hillary collapses or shows any significant evidence of fatigue or physical weakness, this election could well be over.
Finally, even if there is a gaffe, there is going to be an automatic double standards for the candidates that, once again, favors Trump; namely, that Trump can get away with gaffes that Hillary cannot. In fact, gaffes are such a regular part of Trump's rhetorical repertoire that the gaffe is going to have to be a big one in order to be news at all, whereas Hillary will be hurt by even a moderately serious gaffe, since her strength is stability and experience.
And of course this plays back into the expectations game.
Trump has more to gain and less to lose than Hillary, making him the favorite to win.