I think this is PTSD from having lived through a Trump administration.
Just like Dems were coming out in force to vote against Trump last time, they will come out in force to vote against Trump this time as well. Except where Biden was losing support among certain demographics, Harris is gaining support. Where Biden's age and competence was turning off young and politically centrist voters, they have been energized to support Harris.
Harris hasn't even started to define the terms of this contest, which will be centered on things like women's health/abortion rights, workers' rights, and the environment.
I honestly am not worried about Harris's last time being unfavorably reviewed. She was fighting in a crowded field of competent and electable politicians who now have all coalesced behind her. They are already acting as her surrogates. There is party unity, none of this hurt PUMA shit (the Party Unity, My Ass Clinton supporters who didn't vote for Obama because Hillary was skipped over), or the alleged Bernie Bro jive (urban legend that the Bernie Bros cost Hillary the election, when in fact Bernie campaigned for her at 11 different rallies and a higher percentage of Bernie supporters voted for her than Hillary supporters voted for Obama).
The final bit to remember is, there is now some false dichotomy at work here, that Harris hasn't been vetted by the American electorate because she hasn't presented her policy proposals for review, when in fact Trump has almost never talked about any policy in terms other than vague approximations. Harris need only make this election about embracing the future - which is joyful, young, free, female, and multicultural - and moving on from the hate-filled Christian nationalist patriarchy/gerontocracy.
Everyone still needs to campaign and canvas and do what they can to help elect Harris, but we are in a better position now than we were when Biden ran in 2020.