Let’s assume Biden prevails — either by winning enough delegates outright heading into the convention or by losing on the first ballot at the convention but then coming out on top during a subsequent ballot (when the party’s superdelegates are allowed to weigh in). Will voters who favored Sanders accept that outcome and support the former vice president? And will those who preferred Warren or Harris and were unsure about whether they could motivate themselves to work up enthusiasm for Sanders, an old white man who is at least impressively progressive on most issues, get themselves to rally around Biden, an old white man who stands for everything they want the Democratic Party to leave behind?
I’d say it’s far more likely that a significant number of these committed leftists will bolt the party in disgust, looking to throw their weight behind some other, purer alternative in the general election. That would be politically foolish in the short term, ensuring a victory for Republicans. But for those who despise the milquetoast Democratic establishment, it would also be a highly satisfying way to demonstrate that establishment’s impotence to the world — by working to ensure that it fails in its effort to defeat a widely despised president.
That’s one way the Democrats could fracture. Another would involve Sanders prevailing outright or on a later ballot at the convention.