These are estimated based on some survey questions where voters were asked their views on a number of issues that had also been voted on in Congress. As you can see, elected representatives are generally more extreme than voters.
Polarization in red, purple, and blue states
We also looked at these distributions among Republican, battleground, and Democratic states (categorized based on their presidential voting patterns in 2000 and 2004). Geographic polarization is strong, especially in the Senate:
Voters in Republican and Democratic states are slightly more conservative and liberal, respectively, on the issues. Elected representatives are more geographically polarized: winner-take-all elections generally magnify differences that are already there. In a strongly Democratic-leaning state, it is likely that both senators will be Democrats and will be on the left side of the political spectrum. Such a state will also typically have many strongly Democratic congressional districts. The reverse pattern holds in Republican states.