Like other candidates, Sanders’s support has a demographic core: Just as Elizabeth Warren depends on very liberal professionals and Joe Biden on older minorities and moderates, Bernie depends intensely on the young. But his polling also shows an interesting better-than-you-expect pattern, given stereotypes about his support. He does better-than-you-expect with minorities despite having struggled with them in 2016, with moderate voters and $100K-plus earners despite being famously left-wing, and with young women despite all the BernieBro business.
This pattern explains why, in early-state polling, Sanders shows the most strength in very different environments — leading Warren everywhere in the latest FiveThirtyEight average, beating Biden in Iowa and challenging him in more-diverse Nevada, matching Pete Buttigieg in New Hampshire and leading him easily in South Carolina and California.
Now, I have stacked the argument slightly, and left out a crucial axis of division where Sanders does worse than you expect: He struggles badly with his fellow Social Security recipients, the over-65. This weakness and Biden’s strength with these same voters are obvious reasons to doubt the case for Bernie as the unifier, Bernie as the eventual nominee.