There is little reason to believe that Nominee Sanders would cause a split in the dem party. It would be hilarious but illustrative to see how, if sanders is nominated, democrats declare him the best thing since sliced bread. the worship would be universal, however nauseating. It would just be democrats being themselves.
The difference with Bernie is he really believes his "reform" ideas and he'd be extremely dangerous to the entrenched powers. He's like the polar opposite of Pres. Trump, but equally dangerous to the Uniparty.