NZ's top left wing blog The Standard has a post in which the author argues:
One of the great strengths of the Right is that the rank and file supporters are such slavish followers of their leaders. ... Simply doesn’t happen with the Left’s support base. You can’t get the buggers to agree on anything. They care about detail. They argue over it. The educated ones, especially, see simple slogans and shallow arguments as anathema. They don’t tend to go in for blind adherence to their leaders either. They see leaders as tools. Servants for furthering their shared ideals. They critically assess what their leaders say. If they do agree with it when they repeat it to others it will be in their own words. Not a simple repetition of a carefully crafted slogan*. The Left will never have our version of ‘PC’. It’s just not the way we think.Certainly there are often cognitive differences between people on difference parts of the ideological spectrum, but I think any reasonable non-partisan would think that this far overstates any possible case.
But we shouldn't be dismissive - there is a potential evolutionary explanation for why people do things like this.
In group selection, the necessary (and most problematic) part is getting people to irrationally commit to a group - that is, so they stop considering the relative merits of other groups. A group that people left as soon as they found a better one wouldn't be very stable, and would be selected against. Of course, this is hard to do - even if you assign people to monitor defection, they are also incentivised to defect, and if you set up people to monitor the monitors, the problem just regresses! Some have proposed religion as possible solution (God is impossible to defect from if you believe in Him), but another potential one is simply costly signalling (although obviously this is part of the religion story as well).
Costly signalling is essentially just telling people that you are in the group. It has to be costly so it's difficult to fake (and therefore you are less likely to tell people you're committed to the group when you aren't). I think we can look at posts like the one discussed (and in fact a lot of political behaviour in general) as part of a scheme of costly signalling - the author is just telling his/her left-wing friends that they are part of the group, and he/she would never consider joining the other group because they are so deplorable.
Too simplistic? Of course. But I am fairly sure that it is a substantial part of why political behaviour is so irrational.