the breakdown is what determines these elections, who's more motivated?
The Clinton Crime Machine does have a much stronger grass roots system, Trump needs people to just show up on their own accord (of course, that's always been the case)
Party identification also swings more so than people realize, there are many that identify differently from cycle to cycle, so that creates some polling problems although pollsters (are supposed to) ask questions to prevent that problem from distorting their numbers
To your point, one widely talked about poll a couple of weeks ago included 51% democrats and gave Hillary a huuuuuge margin (might have been a debate poll, I'm not sure). They obviously can and do provide selective bias when they choose to do so.
