As to your last paragraph, I think it's more sinister than that. Politicians are people. All people are flawed. All of that goes without saying. I think that the political parties, though, actually corrupt the people who identify with them. I know it's sort of a counterintuitive claim because political parties don't exist without individuals, but I honestly believe that the root of our political unrest has more to do with structural politics than human nature.
The American political system was good intentioned. The founding documents did a decent job for a long time. They can still save us if we amend them, but in their current form, they are largely obsolete. The way people produce, share, and consume information is utterly unrecognizable to the Founders. The career aspect of public service was never intended by the Founders. I have no illusions that the Founders did not fully comprehend and condone the influence of money over politics, but I do not think their wildest imaginations could grasp the relative wealth of modern corporations.
I have never registered with a political party. I try to be consistent in my judgment of them, but their offenses are not always equal. As you and I have both said, we collectively have more in common than not, but that doesn't prevent unlimited money and psychological warfare from manipulating the average, undereducated American into hating anyone who deviates from their thoughtless political ideals.