I met Nixon in the 60's, introduced Jimmy Carter on my college campus when he ran in the 70's, I worked for Richard Riley (2 term Gov. of SC and Clinton's Sec. of Ed.), I met GWB when he was campaigning for his father, and the list goes on and on.
Even though it may be changing, you can't win the South (or even big purple states) like Florida with purely progressive ideals. There are too many independents, too much money spewing propaganda, too many church pulpits preaching a "conservative" message.
Unless a candidate is personally attractive (like Reagan and Kennedy and Edwards) so that they transcend the issues, most folks in the middle are either uninformed or feel strongly about a particular issue. In our current times: women's rights, immigration, and increasingly the environment are the "middle" group issues that will likely drive the independent vote.
I realize that the progressives are obsessed with the bankers, and some special issues (like gun control and health care). I also am interested that reforms are needed. Unfortunately, voter turn out has been dismal, and the independents aren't really unified on particular issues right now. As such, getting crossovers with women and on immigration is one way that Hillary can win. I will vote for the Democratic candidate, but I've seen plenty of "blue dogs" do well in the South when they had something to say that was important to voters. You can go back to Bryan Dorn, Claude Pepper, etc. and see it.
Right now, I don't think either Sanders or Warren would have a snowbird's chance in Florida or SC or GA. Hillary could win FL and might even surprise people in other Southern states.