For starters, the number of Sanders supporters who will actually follow through on their commitment not to vote for Biden is likely negligible. Sure, 15% of Sanders supporters reported that they’d vote for President Trump, but in 2016, only half of the 20% of Sanders supporters who said they’d vote for Trump over Clinton actually did. Only 7% or 8% of Sanders supporters would do so in 2020 if this trend were to persist.
Furthermore, the presidency is won through the Electoral College, not the popular vote. Biden has still retained his dominance over Trump in swing states and counties where Trump and Clinton were within 10 points of each other in 2016, and in the states that do matter, Sanders’s more extreme signature proposals could only harm Biden. The presumptive Democratic nominee has a lead over Trump in crucial Rust Belt states, but could he retain that if he backed the Sanders ban on fracking, an industry that employs hundreds of thousands of Pennsylvanians?
And besides, Sanders’s vaunted “unprecedented turnout” promise never actually materialized, whereas Biden’s winning coalition did.