There is a sizable portion of the country (I’m not about to estimate numbers, I’m not a political scientist) they are neither “honestly progressive” nor “hard right.” They are, for lack of a better word, “moderates.” So long as we have a first-past-the-post system that, when combined with incumbent-protection ballot access rules, makes third parties all but impossible, those moderates have to vote for someone who’s either a Republican or a Democrat.
The problem isn’t with these moderates who hold office and feel disaffected wanting a new home pulling Dems to the right. It’s with a system that says they have to be in either box A or box B. You can’t have a two-party system that consists of only “honest progressives” or “hard right.”