A large part of the electorate DOESn'T vote rationally. It's always been that way. People vote their identity, not their interests.
For example, people who don't make millions of dollars a year routinely vote for candidates who unabashedly put the interests of millionaires first. There are a lot of reasons why a poor person might vote for a candidate whose policies only benefit the rich at the expense of the poor; lots of Americans truly, sincerely believe that they will be rich one day; lots of poor white Americans see poor minority Americans as their enemy, and wish to distance themselves from them; lots of people subscribe to the Just World fallacy, and assume that if someone is successful, it is because that person deserves to be successful. So, time after time, year after year, people vote against their own interests.
People also vote narrow interests over their own well-being. A person who is unemployed and who blames immigrants for his joblessness may vote for a candidate who comes down hard against immigration, even if that candidate's other policies hurt the cause of the jobless (for example, a candidate who comes down against immigration but also supports policies that encourage businesses to outsource to foreign countries).
People also vote their perceptions, rather than reality. People rarely read the laws that affect them; instead, they listen to bumper stickers and sound bites. A lot of folks are familiar with so-called "frivolous lawsuits" and support laws that are billed at ending them, even though genuine frivolous lawsuits are incredibly rare and most laws aimed at abolishing them actually serve the purpose of preventing citizens from suing companies for any reason, frivolous or no. I watched a documentary some time ago that talked to a man who'd been a campaigner in favor of a law in his state that was set to eradicate frivolous lawsuits. The law passed. Later, that man was severely injured by a doctor who, as it turned out, had lost his license to practice medicine for gross negligence and continued to practice anyway. The man attempted to sue the doctor, and discovered that he couldn't. The law that had passed--the law he had campaigned for--effectively barred private citizens from suing corporations at all. The guy had never read the law he went door to door supporting!
Another way that people vote their perceptions has to do with framing. If a particular candidate frames an issue in a way that appeals to the prejudices of his supporters, he can easily get people to vote against their own interests. Examples of this include framing the health insurance reform law as "socialized medicine," even though it's not even remotely close. What we have is basically a law that makes some minor changes to health insurance regulation being treated as though it is a major, radical overhaul of health care in general; since it was framed as "socialized medicine," as ridiculous as that is, people who actually benefit it still oppose it, out of a knee-jerk hatred of anything they believe to be "socialized."
And finally, people vote their prejudices. People who identify as evangelical Christian will tend to vote in favor of candidates who identify themselves as evangelical Christian, even if those candidates endorse policies that hurt the voters (for example, by endorsing policies that financially support companies who move jobs out of the country). If a candidate condemns homosexuality, voters who don't like gays may vote for him even if he endorses policies that hurt those voters.
