The Democratic Party needs to run a candidate that is elected through a fair primary process. When this happens, dems win.
2008 - Obama was popular and won the primary
2012 - same
2016 - It was deemed Clinton’s turn, she was given every advantage, and lost
2020 - Biden won the primary as elected by the people, then won again
2024 - Harris was given the candidacy without having to primary, and lost
Yeah, I probably should've put won in quotes, but I'd argue there's still a small difference between running as the incumbent and the situation with Clinton. Not that it really fucking matters now.