The economy is ALWAYS, FUCKING ALWAYS, the driving force behind the general public's vote. The current American economy, when observed at a macro level, has been great under Biden's presidency. Inflation induced by COVID and Trump's first term policies has been declining, GDP is up, the trade balance is good, and interest rates are starting to come down. Americans aren't benefiting from this in their personal finances. The wealth gap is severe because of trickle down economics. The democrats are shit at communicating this or even having a plan for it. Instead the democrats campaigned that identity politics were as important as the economy. Just fucking stupid. We all know the democrats policy on minority rights and protections. Talk about the economy and how you'll make individuals wealthier.
Some exit polls asked what emotions voters felt when they voted. The bi-partisan answer was overwhelmingly anger. (I think it was a cnn poll and a smaller website poll.) Guess which candidate plays into that emotion and guess which ran on "good vibes and joy."
In 2020, Harris wasn't well liked and didn't primary well. She may have done her job well as VP, but the public didn't know about it. She and her campaign didn't even communicate what she was doing. When she was "anointed," that turned off many democrats. Most of the excitement came from relatively few. The relatively many democrats or people who could be persuaded to vote democrat were not motivated. At current count, Trump has lost about 6 million votes from 2020. Harris has about 14 million less votes than Biden in 2020.
I think there are 3 reasons.
TLDR The DNC are master fuckups