In both cases, those on the left side of the political aisle demonize their opposition in a manner reminiscent of a political campaign where the goal is to prevent an opponent’s election. In short, they seem to see politics as a battle of personalities not ideas. They always need a villain. Yes, I grant this is true for many on the right, especially certain extreme social conservatives.
Why is it they believe they can best advance their argument not by taking apart their opponent’s case, but by taking that opponent apart (or defining someone as their opponent so as to eviscerate him)?
At least in part because they've never learned the difference between truth and propaganda, and believe they are synonomous and interchangable.
Some good stuff in the comments as well.