Why Liberals Lose
It's the ideology, stupid.
In primary contests, why are conservatives more successful at nominating conservatives than liberals are at nominating liberals? There are dozens and dozens of theories about this, most of them focused on something to do with organizational power, but in addition to all the theories, there's an actual answer to this question. Nick Baumann has it here. It should probably be tattooed on every liberal's forehead until we all finally face up to it and figure out how to address it.