Keep in mind that tracking polls track likely voters.
It's erroneous to extrapolate from likely voters to the population at large. Granted that the sentiment of likely voters will line up quite well with the population at large, it's just that a likely voter poll necessarily rejects people who don't give a shit. The people who don't give a shit shouldn't be counted as supporting anything.
538 isn't really any less biased a source than Rasmussen or RealClearPolitics or the LA Times poll. They have a pretty clear agenda, and appear to be drafting on the tailwinds of past successes. They've been dismally wrong throughout the primary season.
Based on what?
Most of these polls (as of today), put Trump at within a few percentage points of Clinton:
http://www.realclearpolitics.com/epolls/latest_polls/