Obviously the FBI, the director of which deliberately grandstanded and showboated throughout the election to an extremely unprecedented degree—and to the detriment of a candidate who barely did anything wrong.
The kind of FBI investigation Hillary was under in though July is the sort of thing that, for our entire history, disqualified potential candidates. Donors wouldn't touch them. Party operatives would tell them the facts of electoral life.
What does it tell you that this didn't happen here? The Democrats weren't shy of candidates; why did the party apparatus and the donors clear the way for Hillary Clinton? Because she's a woman? But if THAT was so critical, why not throw your money and influence toward other capable female candidates the Democratic Party could have put forth.
Debbie Wasserman Schultz should have been one of the picks. She's the architect of the Democrats' losing their chance to recapture the Senate, and (possibly) the architect of the Democrats losing the White House.