Paul Krugman and Congressional Polarization

In Friday’s The New York Times, Paul Krugman addressed what he sees as the disconnect between Republican rhetoric about the welfare state and the distribution of welfare benefits in conservative and liberal states.  Krugman’s piece is one in a series published by The Times exploring the relationship between state ideology and the scope of the social safety net.  This recent discussion has centered–fortunately in my view–on the work of political scientists.  Dean Lacy, for example, asked: “Why do Red States Vote Republican while Blue States Pay the Bills?”, while Larry Bartels addressed this topic over at the MonkeyCage in “The Narcotic of Government Dependency.”  It’s all timely stuff and worthy of continued attention.
What caught my eye in Krugman’s article was the following claim, also referencing (though only vaguely) the work of political scientists:

Modern Republicans are very, very conservative; you might even… say, severely conservative. Political scientists who use Congressional votes to measure such things find that the current G.O.P. majority is the most conservative since 1879, which is as far back as their estimates go.

Krugman doesn’t provide citations with his article, but much of this research stems from the pioneering work of Keith Poole, Howard Rosenthal and Nolan McCarty (see for example here or here).  The data Krugman references can be found on Poole’s webpage: Voteview.com.  Here is the time series from 1879 to 2011 in the U.S. House of Representatives.  I’m using the absolute value of the DW-NOMINATE scores with Congresses on the x-axis:

So is Krugman correct about the modern GOP being more conservative than at any point since 1879?  Yes, according to the data.  At the same time, however, Democrats are approaching their most liberal point since 1879 (the most liberal Democratic cohort in the U.S. House was 54th Congress of 1895).  Moreover, one point I repeatedly make in lectures or class discussions is that polarization is the rule rather than the exception.  We can see in the figure that while polarization is high today, it’s not that uncommon in U.S. history.