"The Comey Letter" Is The Complete Answer To: "What Did Democrats Do Wrong In 2016?"
If it weren't for Comey, Hillary Clinton would have won the popular vote by about 6 points and the Electoral College by 70 or more. And that might have turned into control of the Senate as well, though that's a little more speculative.
This bit from Vox is critical. You can always find grumbling campaign workers. Wisconsin had a surplus this year! What you don't always have is a controlled experiment with 3 "rust belt" states flipping in the last week despite 3 very different efforts from the Clinton campaign. If the campaign strategy is what made the difference, then these 3 states would reveal that. They reveal the opposite:
The appeal of big-picture narratives about demographics, along with anecdotal evidence of big mistakes by the Clinton campaign in certain key states, makes it easy to point fingers. But looking specifically at the three “Rustbelt” blue states mentioned at the beginning of the article, no unifying picture emerges. Most stories mention Michigan, where Clinton didn’t campaign, rather than Pennsylvania, where she campaigned intensely. Indeed, these three Midwestern states (Wisconsin being the third) provide essentially an A/B/C test of different campaign strategies — and in each state she came up just short.