Our friends at the Cook Political Report have released an updated Partisan Voting Index that now includes the results of the 2008 presidential election. They’ve also made a small change in the formula used to calculate PVIs, and I think it’ll be instantly recognizable to SSPers:
To determine the national average for these latest ratings, we have taken the average Democratic share of the two-party presidential vote for 2004 and 2008, which is roughly 51.3 percent, and that of Republicans, which is roughly 48.7 percent. So, if John Kerry captured 55 percent of the vote in a district and Barack Obama carried 57 percent in the district four years later, the district would have a PVI score of roughly D+5. (Emphasis added.)
As we discussed at length, the old PVI formula compared district-level results for the past two presidential elections to nationwide results for only the most recent election. This choice sparked plenty of debate, and some folks even suggested we use our own “SVI” that would compare 2004 to 2004 and 2008 to 2008.
Fortunately, the debate has been resolved. As you can discern from the description above (the key part is in bold), Cook has decided to revise its methodology along the lines proposed by people here. Charlie Cook (an SSP reader, as is House editor David Wasserman) told me he wanted something that was “totally apples and apples,” and I agree with the choice. Ultimately, this means that the new PVIs will be about two points bluer than under the old system – e.g., a district that would have been R+10 will now come in at R+8.
You can find the new PVIs by partisan rank in this PDF, as well as by member name and by state/district. There’s also a giant-size map and a cool chart showing trends in the PVI over the last decade. (As you’d expect, the number of “competitive” districts, at least on the presidential level, has been shrinking.) Have fun!