The Republicans really thought they were going to win in 2012 and I was extremely nervous about it the night before even though Obama was up 3-4 points in the polls. It was a much easier path for Dems in 2012 because the toss-up states were places like Ohio, Florida, and Iowa. The Blue Wall states (Wisconsin, Pennyslvania, Michigan) weren't really in doubt.
That's what sucks about this year. Harris may have a similar national lead in the polls as Obama by election day and may even out-perform Obama as far as national popular vote margin, but she has a much more difficult path to actually getting elected.