Most people perceive their political opinions are the correct ones.
Do you believe that endows your person (or the political movement you belong to) with special moral stature that makes you an exception to the general rules of generalized moral conduct? For example, does it put you, as a person, above showing respect toward people with differing opinions, or excuse you from practicing a high degree of honesty and open mindedness when promoting your own point-of-view?
It seems to me as though a lot of people aren't very rigorous about testing or challenging their own beliefs because they perceive these beliefs give them some sort of special privilege, where respect and honesty are not virtues they need to develop and practice because advancing your political agenda comes first.
That explains a lot of the behavior in politics.