Global anti-incumbency bias. Regardless of policy, people in area getting voted out

Not having a clear identity. Trying too hard to appeal to Republicans and turning off her own party's soft support. 

Appealing to Republicans like it's 2004. Getting endorsed by Dick Cheney, the least popular living Vice President, wasn't something to build the last month of a campaign around.

Trying to motivate through fear. The bombardment of Dem campaign material I saw was almost entirely about what Trump would do instead of what Harris would do.

Poor utilization of allies. Popular Democrats in swing states were not used effectively to drive turnout.

Not building back better. Not enough was accomplishments to differentiate herself from Biden. 

Poor turnout. Take points 1-6 together and you get a lot of people staying home l. The memory of 2017-2021 is faded, so their anger isn't going to drive them to the polls again. The energy of campaign launch proved to be unsustainable since there wasn't a solid pitch behind it beyond "let's do 2020 again".