Then, Sanders should make clear than party loyalty is important and make float the idea of a primary challenge to every Democrat office-holder backing someone else than the official candidate.
Hillary will have no choice, but to follow suit, given the pressure.
1)A guy who isn't a registered Democrat is hardly the ideal person to talk about party loyalty.
2)Nobody will overtly support Bloomberg, they will just sit on their hands. After all, Sanders has raised 0 dollars for state parties and Dem congressional candidates, so it's not like they owe him anything.