Up through the 1970's, I can remember having lively yet respectful debates with my liberal friends on a host of political issues. After that time, such debates became less and less frequent and more likely to end up in acrimonious responses to the same questions I had previously posed. Nowadays, it is extremely unlikely to find anyone of a liberal persuasion who is willing to discuss any political issue without immediately resorting to personal attacks.
What happened to these people? My theory is that, essentially having won the debate over the size and scope of federal programs, they are now defensive about the lack of societal benefits these programs were supposed to achieve and are seeking to impose ever more radical "solutions" on a recalcitrant public in order to achieve them. In this vein, anyone with contrary views is considered a saboteur of their good intentions.
What is you theory?