Candidates use psychological strategies to win support and to cast their rivals in a negative light. In projection, people seek to lessen their own faults by calling out these faults in others.
Because everyone in the GOP in so mentally and ethically bankrupt that all they can muster is constant projection of their own worst ideas and instincts out into the world.
"If I think I should do this awful shit, I guarantee they're going to do it as well." - GOP rank and file
The only quibble I have with this is that, as the article points out, projection is generally a subconscious things people do. I firmly believe Trump knows exactly what he's doing and he accuses others first so if they accuse him it looks like "no, you are!" It's just one strategy in his con man book.
Narcissists often end up in therapy when divorce papers are served or they are fired for abusive behavior. They can’t see these behaviors until forced to confront it: the entire set of behaviors is specifically a self defense mechanism to cover over their insecurity. 100% Trump believes everything he says.
If you drag everyone down to your level, everyone looks bad, or at least you don’t look as bad by comparison and the voting public loses confidence in the system in general, which tends to benefit fascists.