Via Dylan Matthews, here's an interesting set of charts from a pair of researchers who studied how people feel about various issues vs. how politicians think they feel. The left-hand chart, for example, shows opinions about universal health care. In districts where 50 percent of the voters supported it, liberal politicians estimated that only 45 percent supported it, while conservative politicians estimated that only about 30 percent supported it. This same gap shows up across the entire spectrum, and for a wide variety of issues.
As always, this is one data point from one study. But it certainly seems to suggest either that conservative voters are a lot louder than liberal voters or that politicians themselves simply don't understand how quickly some of these attitudes are changing. Needless to say, if these results are generally true, it tells us a lot about why it's so hard to get liberal legislation passed.