In the past there has always been a "bandwagon" mentality to these polls. In the past worked a lot better when all we had was our local newspaper and the 3 evening news channels (CBS, ABC, NBC).
People realize that nowadays. Plus the fact, I'd wager that 1 in 4 Trump supporters are wary of sharing that view due to the SJW backlash, doxxing, etc. My feel is that this thing is razor close, but Trump has the momentum.