The more I think about it the more I'm convinced her problem was Biden. The Democratic senators that are winning in the swing states spent an entire year separating themselves from Biden.
Harris didn't get that chance. She took over Biden's campaign, his advisors, his ground network, his convention, 4 months before the election. If he had just recognized his time was over we could have had a real primary, with different campaigns to find the one that resonates with the people.