They come up with ever more attenuated arguments that, while being clever on their face, end up ignoring basic realities of the human condition and as a result lack any common sense.

That's combined with a growing ignorance on the part of the rank and file, though. I used to have interesting debates with people on the Left because the issues would be regarding the nature of the social contract, to what extent self-reliance and government assistance, respectively, should be driving forces in policy, etc. Now it's devolved to name-calling - I'm a Nazi (okay, fine, people have said that since middle school), I'm an oppressor with privilege blah blah blah, and so my points are therefore totally worth ignoring.