In a nutshell, the Democrats can’t convince people to vote against the dangerous candidate because right-wing populism inoculates people against facts and logic by making those things out-group markers, per se. Identity is powerful, and the human brain treats threats to identity in exactly the same way as physical threats.

And, on the other side, Democrats can’t recognize this and respond appropriately, because they’ve made not-recognizing-it a marker of in-group identity, and they are thereby unable to decode what would make an attractive policy plan.