That is of course the whole problem. Those holding Charles' view on this subject believe the issue is settled and so beyond party politics. It's like claiming belief in or opposition to the law of gravity is a party politics issue.Those remarks were rankly party-political
In fact, most liberals believe that all liberal views are not partisan political issues, they're just the way all decent people think. Only positions opposing them are considered partisan in nature.
This POV is, for most of them, entirely genuine. They really believe it.
Why wouldn’t they believe it? They are in the middle of a mutual admiration society; everyone they know agrees with them, and they agree with everyone they know. Going along and getting along with the propaganda media is their religion.