When the story of this election is finally written I think overconfidence by the democrats will be a big part of it, from rolling over on the Merrick Garland nomination, to Obama silencing the FBI on Russian State interference. You can see how they got there in every case, but all of it was built on the faulty assumption that Clinton would win, and so there was no need to take political risks.
How much of that assumption was overestimating Clinton and how much was underestimating Putin remains to be seen.