The Cook Partisan Voting Index
The Cook Political Report’s Partisan Voting Index attempts to measure the competitiveness of each U.S. congressional district by comparing the presidential vote outcomes against the nationwide results for the 2004 and 2008 U.S. presidential votes. “A Partisan Voting Index score of D+2, for example, means that in the 2004 and 2008 presidential elections, that district performed an average of two points more Democratic than the nation did as a whole, while an R+4 means the district performed four points more Republican than the national average. If a district performed within half a point of the national average in either direction, we assign it a score of EVEN.” Of 435 districts, 222 — more than half — are Democratic or Republican by more than 10 points;. Even so, this is only a reflection of the presidential vote; a strongly Republican district in this sense may have a Democratic representative, and vice-versa. Via The Electoral Map.