He has no natural constituency in the Dem party, except maybe those who would take an appearing centrist just to win. Most of those people have fled since the Carter years.
The best chance to put in a pretend centrist boring white guy would be Evan Bayh, who bailed out at a good time from Indiana.
IIRC he is against affirmative action so that means 40% of Democrats will hate him automatically. He has no path.