Michael Kinsley, commenting on disagreement in political discourse:
It's a free country, and people can believe whatever they want. If evidence or reason persuades them that some opinion they hold is wrong, they are free to change it. So at any given moment, we all believe that our own beliefs are correct and anyone who disagrees with us has some explaining to do. Furthermore, if I believe that evidence and reason support my own views, then I also must believe that they do not support the views of those who disagree with me.
Three possible answers are that they are misinformed, they are thinking poorly, or they are blinded by self-interest.
This completely misses the idea of differing values, which is among the most common reasons for political disagreement. Jonathan Haidt has done some interesting work on this issue, which is highlighted in his 2008 TED talk. He found five foundations of morality across cultures.
Liberals care much more about Harm and Fairness a lot more than Authority, Ingroup and Purity while conservatives care about all 5 channels equally. These are fundamental differences and bipartisanship or even political discourse in general is very difficult as long as people pretend that everyone wants the same thing and the only disagreement is on how to get there. Politicians playing to the median voter can pretend that they care equally about every moral value, but responsible commenters should be less biased. At the very least, pretending that the only possible way their values differ from others is that they are less selfish is not constructive discourse.
An example of another major value premise difference is time preference, where two parties can reasonably disagree on the proper discount rate. For this premise, it can be a little bit funny watching the debate shift, because liberals discount the future benefits to economic growth but don’t discount the damage to the environment while conservatives do the opposite. If pressed, liberals might say they don’t believe that their regulatory or redistributive policies will slow economic growth while conservatives against environmental policies believe that environmental protection done incorrectly hampers growth and as long as there is economic growth their descendents will be rich enough to deal with any issues that come up.
Of course, it is also true that voters from both sides of the political spectrum are drastically misinformed.
Hat tip: Arnold Kling