Partisan polarization and a radical decline in ticket-splitting means that down-ballot Democrats will have little incentive to abandon their presidential candidate even if they think he can’t beat Trump in their areas. And just as importantly, today’s Democratic Party and its constituent elements are a lot closer to Bernie Sanders than they were to McGovern in 1972.
If you get too caught up in today’s “warring factions” of the Democratic Party you can forget that it used to be far, far more diverse ideologically. McGovern was dealing with a party that still had hosts of open segregationists, Cold War militants, law-and-order enthusiasts, and culturally conservative Catholics. His Democratic Party was still divided over the Vietnam War. The famous claim that McGovern was the candidate of the “three A’s — acid, amnesty [for draft evaders] and abortion” wasn’t invented by Nixon’s dirty trick artists, but by fellow Democrats (including his future running mate, Thomas Eagleton).
And most notably, McGovern’s abandonment by the Democratic Party was exemplified by the exceptional hostility of the labor movement. The AFL-CIO was neutral in the 1972 general election for the first and only time since it was formed. That’s not going to happen to Bernie Sanders in 2020.