Lastly, people really are fucking stupid. I know this because I'm smarter than most people and I do stupid shit. If even bright people are doing stupid shit that they shouldn't be doing, ordinary people; including that half the of the population which is below average intelligence, is doing a lot of stupid shit. Especially with regards to issues about which they're poorly informed and feel alienated from. Which leads us right back to emotions.
I will continue where Cyncat left off here. I am smarter than not just most people, but the vast majority of people, and I still do stupid shit (less now than when I was younger, but when I fuck up, it is catastrophic given the context. Especially socially). Ask Cyncat, Hotfoot, and Frigid. They have stories. I also have certain issues I get worked up about and sometimes suspend aspects of critical thinking.
But there is other shit to. Natural cognitive biases that affect people's decision making. Like plain old repetition. If something gets repeated to you often enough, you often buy into it. So there are people I know who are normally very sharp clever people who i hold in high regard, who buy into some of the BS about US healthcare reform (I hate it, but I hate it for the right reasons), or who buy into republican bullshit about taxes and job creation. Why? Because they hear it so often that their brain concludes it must be true. No one is immune to this. I repeat, no one is immune to this.
There is the alpha male thing that Cyncat mentioned. But also, people are more likely to support those who look and act like them. We are apes. That person who looks and acts like us is a member of out tribe, and we dont trust that person from the other tribe. So Bush did really really well in elections, because he could fake being a member of the Blue Collar Red-Neck tribe really well, despite never actually working a day in his life.
There are a few more, like people not actually having a very good grasp of statistics. They are more likely to fixate on one case, than what happens statistically. This is why many americans fixate on medical horror stories in single payer systems, but ignore superior overall performance. It takes actual training to reliably avoid this one.
There is also cherry picking of information to agree with pre-existing biases and preferences. This is called confirmation bias.
Also, Batman: I am in a similar position to Havoc. I dont have a lot of time to donate, but I donate money, I write letters, and I do everything I can do within professional ethics and politeness to convince people to vote certain ways. That includes trying to prevent the erosion of our civil liberties. So, if you could try to not paint Americans with too broad a brush, that would be awesome.