The method of propaganda is to feed your existing biases, then redirect them in the desired direction. Pace, then lead, as the Neurolinguistic Programming folks put it. Propagandists appeal to the affective element of your personality, while convincing you–the target– that the cognitive, rational element is irrelevant, and unimportant.
When you see the claim that “liberals” are governed by emotion, this is the actual reason. They are dupes of effective propaganda. This happens everywhere of course, but is more common on the Left for the simple reason that their policies don’t work, so they need to be sold on levels other than the rational.
This does not make Conservatives rational per se, but what I have found is that I can defend conservative ideas, and cannot begin to defend Leftist ideas. There is a middle, but that middle ends when rational dialogue ends.