Reminds me of Thomas Kuhn's "The Structure of Scientific Revolutions". His argument is that really big changes in science come about when the upstart theory wins over the upcoming generation, since the older, established, scientists can't change their views radically.
Professional historians have largely been fairly liberal for some generations now. Since they are the writers of serious history, it has been their views that have come down. If they somehow train up a generation of right wing historians, that could change, but not until.
Of course, one has to wonder why those whose profession is the study of history tend to have liberal views. Is there something in the detailed study of what has happened in the past that leads to such a position? Or is it just that those with other views tend more to spend their lives in other fields?