How Tuesday’s elections made Congress more polarized, in one chart

November 8, 2012

When you want to get quantitative about the policy views of members of Congress, your first stop should always be DW-NOMINATE. The scale, which uses Congressional votes and other data to place representatives and senators on a left-right spectrum, was devised by Keith Poole and Howard Rosenthal in the early 1980s and has been making data nerds happy ever since.

Now, Poole has calculated the expected scores of the 113th Senate, which was elected on Tuesday. Overall, the chamber is becoming more polarized:


Source: Keith Poole

For one thing, the average Democrat is slightly more liberal and the average Republican significantly more conservative, meaning the typical members of the two party caucuses will be farther apart ideologically.

But most striking in the above chart is that the two curves no longer intersect. That's partly an artifact of the graph type — intersection doesn't necessarily mean that there are Democrats who are more conservative than some Republicans, just that the most moderate members of each group aren't that far apart.

But the new curves also signal a real change.

Before this Congress, there were a handful of Senate Republicans who were more liberal than the most conservative Democrats. In the 111th Congress (2009-2011), for example, newly converted Democratic Sen. Arlen Specter (Ill.) was more conservative than Republican Sens. Olympia Snowe and Susan Collins of Maine, and Democratic Sen. Ben Nelson (Neb.) was only imperceptibly more liberal than Snowe.

Now, the Senate is polarized entirely, such that even the most conservative Democrat is noticeably more liberal than the most liberal Republican.

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Dylan Matthews · November 8, 2012