Carter was up on Reagan by 4 points in mid October.
And got creamed.
Close elections are always hard to predict because of the variables involved. As mentioned before, the electorate isn't in love with either candidate. Sure, each has their small base of adoring fans, but most Americans detest both as shown in the favorability polls. If 2% less voters show up on one side and 2% more show up on the other, especially in key states, the entire election could turn into Trump win or a Hillary landslide.
Voting Turnout Statistics - Statistic Brain
Voter Registration Statistics Data
Total number of Americans eligible to vote 218,959,000
Total number of Americans registered to vote 146,311,000
Total number of Americans who voted in the 2012 Presidential election 126,144,000
Percent of Americans who voted in the 2012 Presidential election 57.5 %