TPM has a writeup of a study saying that lawmakers, both liberal and conservative, assume that their constituents are more conservative than they actually are. The summary of the actual study does say "voters" and not just "constituents", which is an important thing to clarify. But the summary also suggests that the reason for this might be that "politically active citizens tend to be wealthier and more conservative". As a remedy, the summary suggests that "progressive groups might be able to use a simple lobbying strategy - just let legislators know the truth about what their constituents think and want!"