Dial down identity politics. Most people are put off by it, especially when taken to the extreme as has-been the past few years.
It's ok to disagree and/or hate policies of the opposition - but it's not ok to label opposition every *ist word you can think of. This pushes people away.
Stop trying to be "Kingmakers". Harris was anointed by the party's elite class and everyone was told to get in line. People don't respond to that well either.
Lastly - run an actually decent candidate FFS. Literally zero people wanted Harris as the candidate - but Democrats had no other choice but to vote for her in the election. This caused Democrats to suppress their own electorate. Coupled with rhetoric such as "we're saving democracy" etc (see gaslighting above), and the results speak for themselves.