Polarization is commonplace in American politics. Both parties are moving away from the middle. The debate often boils down to who is polarizing the most/fastest.
New York Times opinion writer Peter Wehner sparked an interesting debate when he claimed, “In the last two decades, the Democratic Party has moved substantially further to the left than the Republican Party has shifted to the right.” Jamelle Bouie’s excellent response at slate essentially illustrates that a lot of political science does not support that claim.
Both make good points about polarization in American politics. However, in many respects they are talking past one another. Wehner’s piece highlights the issues Democrats adopted in recent years. And as he points out Democrats are undoubtedly more liberal on gay rights, drug legalization, and climate change, among other issues. Bouie, on the other hand, bases his argument on measures found congressional research. He highlights that recorded votes illustrate a much different picture than the one Wehner paints. Despite some excellent points, there is an important dynamic between the issues parties champion and votes on the House and Senate floor that neither author addresses.
The argument for asymmetric polarization originated from recorded votes taken in Congress. Among the first to point this out were Thomas Mann and Norm Ornstein, who argued asymmetric polarization is obvious and primarily occurs on the right. Over the past four decades the average Republican’s vote score has unquestionably moved farther right than Democrats have pulled left.
(See chart and data from voteview.com, the original source of congressional roll call scoring.)
But this isn’t the whole story. To grasp asymmetric polarization you have to understand how roll-call votes occur. And in this context we see a much muddier picture.
Roll-call votes are both a very good measure of polarization and a clearly biased sample. They’re biased toward the decisionmaking of the representatives or senators that control the floor agenda. If a bill can reach the chamber floor, it can receive a recorded vote. If it can’t, then it doesn’t. And to oversimply the legislative process into a single sentence: Party leaders dictate the agenda. Polarization found in roll-call scores is in large part a function of party leaders’ policy priorities, political necessities, or electoral incentives. They can prioritize or deemphasize divisions between the parties depending on the interests and incentives they choose to pursue. As several scholars point out, today party leaders are far more interested in dividing the parties than they have been since the early 20th century. (See Frances Lee’s "Beyond Ideology" for a great description of this.)
The issues majority party leaders bring to a vote affects polarization and how far left or how far right they diverge. So the question we should be asking is: what issues are party leaders deciding to vote upon?
Laurel Harbridge recently tackled this question in her book, "Is Bipartisanship Dead?" She finds that since the mid-1970s party leaders having scheduled more partisan votes. Despite the fact that bipartisan legislation is still introduced with impressive frequency, those bills receive roll-call votes far less frequently. A statistic that really sticks out in her research: In 1974, more than 70 percent of all roll-call votes were on bills cosponsored with a significant amount of members from both parties. By 1995, just 27 percent of roll-call votes had significant bipartisan support. Congressional leaders increasingly bypassed bills with bipartisan support, often for partisan alternatives on the same issue.
If you take this agenda effect with other studies emphasizing how procedural control and votes influence polarization (research done by Smith, Carson, Crespin, Madonna, Roberts, Theriault, and others), there is a body of evidence suggesting that the issues and bills party leaders prioritize are a significant culprit in congressional polarization. And further, Republicans are prioritizing partisan bills and votes with greater frequency than Democrats.
This isn’t wholly surprising or even illogical. Republican congressional leaders have much greater incentives to pursue partisan policies. The Republican base – with hugely safe, homogenous districts coupled with the money, organization, and impressive influence of right-wing Super PACs and groups with the power and willingness to primary moderates – is simply not mirrored to the same extent on the left. 
So while the 111th Congress passed a huge swath of Democratic priorities, on the whole Republicans have more incentives to vote and pass partisan bills. And as a result, vote scores illustrate Republicans have been more prone to move away from the middle than Democrats over the last few decades.
There are a lot of reasons both parties are moving away from the center. But despite the fact Democrats take more liberal stances than they have in the past, those stances receive less attention on the chamber floors relative to the more conservative positions in the Republican Party.
 In this vein an interesting test for Republicans will come in 2016. Speaker John Boehner and Majority Leader Mitch McConnell have prioritized several significant bipartisan bills in recent months. This tactic has illustrated a willingness to govern but runs counter to the tactics employed the last four years that brought Republicans electoral success.
Joshua Huder publishes his Rule 22 blog at http://rule22.wordpress.com.