In one of Progress Illinois' first feature stories, we examined how demographic shifts and Republican missteps both locally and nationally have slowly turned Chicago suburbanites away from the Grand Old Party. With favorite son Barack Obama serving as the Democratic Party leader, we figured the exodus would only accelerate. According to the newest release of the Cook Partisan Voting Index, a measurement that assesses how strongly each congressional district leans toward one political party compared to the nation as a whole, that's exactly what has happened.  Here are the numbers from before and after the 2008 election data:

As the above numbers show, Democrats exhibited strong gains in 2008 in every suburban battleground district.

Rep. Peter Roskam's 6th District is now evenly split between the two parties, a gain of three points on Cook's scale. Rep. Mark Kirk's 10th District fell two points further into the blue, an encouraging sign for the Democrats should Kirk run for higher office. And while three western suburban districts (8th, 13th, and 14th) all boast a one-point Republican advantage, Democrats gained four points in each. In fact, the only districts where Republicans gained any ground were in Rep. Phil Hare's western district, Rep. Bobby Rush's Chicago district (where Democrats still dominate), and three districts in Central and Southern Illinois.

(H/T Prairie State Blue)