The one feeds the other. Do you have any good friends you disagree with politically? Money quote:
Real political debate is mostly done by poor uneducated moderate non-voters with little political knowledge and few social contacts, talking with people they hardly know and rarely talk to. I suspect causation goes both ways here - not only are rich, educated, social, political folks more likely adopt the political opinions of their close associates (over distant others), but such folks also tend to avoid close association with those who disagree politically.