It keeps widening, according to this National Journal story.
Over the past eight election cycles, a great sorting of the House has taken place. Congressional districts have become more strongly Democratic or Republican, and voters across the country have been sending representatives to Washington who are more strongly liberal or conservative, according to a new analysis by National Journal. From 1994 to 2008, voters became less willing to tolerate representatives who were out of step with their ideological preferences, and lawmakers responded by voting more in sync with their districts.
The authors argue that the ideological homogeneity that results from redistricting (the creation of districts that are solidly democrat or republican) makes it easier for errant representatives to either be kept in line, or replaced, by constituents. The partisan realignment of voters based on party ideology also contributes to this polarization.
