They spent it running ads and stuff, Musk went ahead against the taboo and legalnrestrictions against buying votes/voter registeration and did a million dollar a day lottery for registered republicans in a swing state.

I'm assuming whatever spending advantadge Dems had was more or less squandered the moment those billionaires got cold feet and decided to oust Joe Biden instead of relying on the incumbent vote. That disruption very likely scared off several donors.

But yes, there was also simply a lack of energy in the D voter base in 2024. No amount of spending can recover disengaged voters who simply dismissed everyone as "equally bad".