The whole "liberals are mentally ill" thing leaves a bad taste in my mouth, and a few days ago I finally figured out why. The whole concept is teetering dangerously close to an ad hominem.
The pattern goes like this: observer notes Liberal 1's views on subject X. Per observer's views on subject X, Liberal 1's views don't make any goddamned sense. Observer likewise has issues with Liberal 1's views on W, Y, and Z. Since clearly (by the subjective observer's logic) Liberal 1's logic is broken, only a crazy person would persist with such views upon receipt of the fact.
Since aformentioned Liberal has clearly been identified as batshit fucking loco, logically it follows that any person that holds such views and/or self-identifies as liberal must likewise be a few fries short of a Happy Meal, or whatever the hell macrobiotic worker-owned cruelty-free restaurants are paid by the welfare state to give to kids.
Observer stumbles later upon Liberal 2. Liberal 2 has been identified thusly because of their opinions on subject W. However, their opinion on subject Y actually makes a certain degree of sense. However, we've established that due to their liberal views on W, anything Liberal 2 says is New Age Marxist word salad, and not worthy of consideration.
Of course an objective observer will simply agree with Liberal 2's point, but subjective observers, which represent everyone but Lt. Commander Data, may (and I admittedly paint with broad strokes) be influenced by a preconceived notion that whatever liberals say is feel-good cods wallop.
Swap 'liberal' for 'conservative' and insert 'Bible-thumping,' 'Gun-toting,' 'Heartless,' or whatever where appropriate, and oddly enough both sides start sounding the same at the facile, identity politics level.
I won't deny that there are people that hold 'liberal' views that could very well be diagnosed with something unpleasant (or 'libertarian' or 'conservative' views, for that matter), and I have no problem with a well-earned fisking, but the key word is well-earned. There's enough real idiots out there that it's not necessary to wrap everyone that disagrees with you in straw and fetch one's trusty scythe.
MattG was a lot less clumsy about this a while back, really.