Feingold on the other hand is still an unapologetic liberal
Why would he apologize for that? And it's not as if Wisconsin is throwing him out for that reason, although that doesn't mesh well with your talking points undoubtedly. Some of the votes he cast in this last congress gave an impression of him as a generic democrat, instead of someone who stands up for what he thinks is right. Wisconsin didn't have a problem with his liberalness for all these years, and they still don't, but they felt he had sold out to the Washington Democrats. That is much more toxic than being called a librul.