The Dems abandoned the working class and downtrodden. Ran a terrible campaign and Harris was flat out a bad option that would always suffer from incumbent dissatisfaction.
Trump connected on issues that affected a majority of people. The lefts lurch towards gender and identity politics alienates more voters than it attracts, huge mistake and waste of resources imo.
There might be an argument for a different Dem candidate having had a better chance of beating Trump, but that is only conjecture and begs the question: Who else but Harris?
That's the thing: Yes, based on the obsession with identity politics that the Democrats have cultivated over the past 15 years, there was absolutely no way they could have shoved the black, female VP to the side in favor of a white dude like Newsom or a white chick like Whitmer. So they had left themselves pretty much no other option than to default to Kamala.
BUT: even without this identitarian trap, they had no truly convincing candidates at their disposal this year. Their party leadership is decrepit and decided to ossify while in office on top of that (Biden and Pelosi in particular), stiffling the development of younger candidates. Bernie and Warren are too geriatric, their window has closed. AOC is too young and polarizing, Michelle Obama doesn't want it, Barack Obama is termed out. Shapiro and Whitmer are emphatic proponents of technocratic status quo politics, which wasn't what the elctorate was looking for this year.
My bet is that the Dem primaries for 2028 will come down to Whitmer, Shapiro, Raphael Warnock or Wes Moore.