For example, Senators who genuinely do believe that carbon dioxide emissions are contributing to a global climate crisis seem to think nothing of nevertheless taking actions that endanger the welfare of billions of people on the grounds that acting otherwise would be politically problematic in their state. In other words, they don’t want to do the right thing because their self-interest points them toward doing something bad. But it’s impossible to imagine these same Senators stabbing a homeless person in a dark DC alley to steal his shoes. And what’s more, the entire political class would be (rightly!) shocked and appalled by the specter of a Senator murdering someone for personal gain. Yet it’s actually taken for granted that “my selfish desires dictate that I do x” constitutes a legitimate reason to do the wrong thing on important legislation.Part of this, I think, is due to a defect in human nature: It's well-known, for example, that while people can instinctively grasp that, say, pushing someone in front on a runaway trolley is wrong, they become less certain if asked to divert a runaway trolley in order to save five people, though it would mean killing another person. In other words, once you take moral considerations out of the realm of the immediate and visceral, it becomes easier to weasel your way of commitments that you would otherwise honor. Yes, you could argue, I understand that carbon emissions lead to global warming and thus to increased suffering for much of the human race; but I'm not alone in contributing to the problem (look at China!); and I can only do so much to fix the problem; and I must look after the interests of people actually existing now, not potential future people; etc. One reason why I think Hurricane Katrina was a turning point in how Americans think about global warming was that it drove the point home for many that global warming is not an abstract issue, but something that threatens our way of life.*
Matt's other point, about the rather petty nature of most politicians' self-interest, is a harder nut to crack. I'm inclined to believe that the job of politician (as opposed to the jobs of activist, advocate, wonk, etc.) tends not to attract people with idealistic personalities -- the kind who set out to accomplish something and get in the history books. That is, to be an effective politician -- the kind who wins elections -- you need to be, first and foremost, a good salesman. If you happen to have an earnest belief in the ability of government to make people's lives better, that's well and good; but it's not a requirement. I don't know if that's a good explanation, but my sense is that the notion that people enter politics out of a desire for making lasting change is off the mark in many cases.
* Standard disclaimer: No, Katrina was not "caused" by global warming, but global warming does create circumstances in which Katrina-like storms become more prevalent.