Last week, I wrote up a post describing how the parties in Congress have polarized in part because they represent more ideologically distinct districts and states than they used to. I produced a chart showing how the states themselves are polarizing; to the extent senators are simply representing their states today, that would lead to much more partisan behavior than it would have a few decades ago.
Here's the same sort of chart showing the presidential vote by congressional district. It's a similar story. The red columns show the number of congressional districts in the 1968 presidential election; the blue bars represent the number of districts in 2012. What this suggests is that congressional districts have become more polarized over time. Notice that the red distribution has a higher peak in the center, while the blue distribution is more spread out on the edges. Today, there are more liberal and conservative districts, and fewer centrists ones, than there used to be.
1968 data from Marty Cohen. 2012 data from Daily Kos.
Interestingly, though, the polarization of the districts doesn't nearly explain all of the polarization in congressional voting behavior that we've seen over the past few decades. Note that there are still plenty of moderate congressional districts, but they're increasingly being represented by liberal Democrats or conservative Republicans. There just aren't many people in Congress voting in a consistently moderate fashion.
Why is this happening? Partially, it's because ideological groups within the parties have become more active in selecting the sorts of candidates they like (usually ideological extremists themselves) and giving them advantages in primary elections. Relatedly, incumbents know that those ideological groups are powerful, and they want to please them to the extent it helps them avoid a primary challenge. Additionally, the parties themselves have become polarized along ideological lines in a way that they weren't in the mid-20th century, when you had liberals and conservatives in both parties. (Remember that Strom Thurmond and Jesse Helms used to be Democrats.) The end result of all of this is that elected officials tend to be considerably more ideologically extreme than the districts they represent.
Does that mean that elected officials are basically robots who just vote the party line? No, far from it. Note the recent vote in the Senate on handgun background checks. Yes, most senators voted with their parties, but enough defected to make the vote interesting, and those that did defect were largely doing so because of public opinion. That is, Democrats in moderate states were worried that gun owners would be outraged and turn against them in the next election, while some Republicans in moderate states were concerned that their voters were still moved by the events at Sandy Hook Elementary and wanted to see some legislative response. And wasn't it interesting that President Obama and many members of Congress switched their views on same-sex marriage at almost the exact same time that the median voter in the United States did?
All of this to say that members of Congress, despite having polarized considerably in recent decades, are still overall quite sensitive to what voters want. You can get a good sense of this by looking at the graph below, created by Simon Jackman, that plots out the presidential vote in each member's district against that member's "ideal point," an estimation of his or her ideological preferences based on how he or she votes in Congress. As the graph shows, there's a big difference between Democratic and Republican voting behavior, but members are still responsive to their districts, with members from more centrist districts voting much more moderately than their colleagues from more extreme districts. Democrats from very conservative districts are voting almost (though not quite) the same as Republicans from relatively liberal districts.
Elected officials are much more partisan than they used to be, but they're still terrified of alienating groups of general election voters. They know that the wrong vote on a controversial topic can be very costly in their next re-election effort. Legislators can get a lot more partisan—California's state assembly members are pretty much ignoring district sentiment in this graph. That's probably where Congress is heading, but we're not there yet.