To answer Tom's question, I think there are various reasons which apply to different actors. One is a high degree of regional tribalism - there are of course many progressive Southerners and they are wedded to the idea of turning their own tribe Blue. One is the influence of BoboBrooksian thinking on many people - the South is real America. One is an understandable unwillingness to abandon a region to the forces of darkness. And, of course, one is the need to both brand and shift the Democratic party to a mythical center which is whatever those arguing for that center claim it is - it's easier to cherry pick that if you have more ideological and regional variety in the caucus.