Jeb only barely carries the deep south
I doubt he even does that. Black people generally aren't huge fans of the Bushes. With no Biden/Clinton/Obama in the race, only a Jeb!, Sanders probably would have actually won the black vote.
Instead, Jeb! does best with some moderate white Democrats in states like Virginia, but that's probably not enough for him to win any state.
MAYBE Florida, as he does have ties there and it's not a great fit for Sanders, obviously.
How badly he loses really depends in large part on why he changed parties and what his new platform is, but even a Jeb! who basically has the same platform as Biden I don't see having a chance to actually be the Democratic nominee. Too many voters have too many bad memories of the Bush years. Plus he's just not a great candidate, being so introverted and awkward and cringey.
