This is largely, I think, a function of our two party system.

"My party is the good one, so anything that seems bad for my party or its politicians must be lie, or blown out of proportion, or not really that important. Because if it is a big deal and it isn't a lie, that will help the other party, and the other party is evil, so that can't possibly be the case." [proceeds to put fingers in ears]

The result is cognitive dissonance that I think is truly harmful