I don’t see any need to drag the discussion of free will versus determinism into this. I don’t mean to be offensive but to me ,the connection is not clear and the arguments are not convincing. I happen to have a different perspective on this issue, as I imagine every thinking human being does.
Still you make a good point about the Leftist idealism (which you call rationalism). Leftist act as if they lived in an ideal world where people behave rationally. Therefore Leftists are not rational, they are idealists.

Physiologically the “first brain” also called “the reptile brain” is responsible for the fight and flight response. The reptile brain has the power to control the cerebral cortex. Only when emotions are handled can humans act rationally and intelligently. It is idealistic, irrational and unintelligent to think that most humans are rational.