Read this story from the NYT about Accountability Now and then read the Sam Stein story on the same subject. The latter is much more accurate.
There's a difference between Democrats from more conservative districts who, while not representing my political views, do manage to represent their constituents' views, and Democrats who engage in "centrist" wankery despite representing liberal districts, or Democrats who use the conservative tilt of their district as an excuse to get on board with anything the Chamber of Commerce supports.