They’re easy to understand: they are privileged and insulated enough from potential Republican fuckery that if Democrats lose elections, they are mostly going to be ok. They’re mostly middle class or higher with secure employment or other economic support (parents, spouse), straight (or closeted), their religious status isn’t contentious, mostly they’re male.
The other group I’d expect would be extremely low information voters who believe all politicians are the same and it’s a Coke or Pepsi kind of thing. That’s not going to be super common with anyone identifying as “progressive” due to the very politicized nature of that term, though.