In 2016, Trump won Michigan by ~11,000 votes. He won Wisc. by ~23,000 vote. And he won Penn. by ~44,000 votes.
In the primaries, Sanders got 570,000 votes in Wisc. and ~599,000 votes in Mich, and ~732,000 votes in Pennsylvania.
Supposedly, 12% of Sanders supporters in the 2016 election ended up voting for Trump. (
https://www.npr.org/2017/08/24/5458...voters-ended-up-supporting-trump-survey-finds)
Assuming Wisc. and Mich. follow that pattern, it gave ~68,000 more votes for Trump in Wisc. ~72,000 more votes in Michigan, ~88,000 more votes in Penn. than if those Sander supporters decided to stick to the Democrats, far above Trump's margin of victory. And had Hillary won those states, she would have become president.
So basically the unhappy BernieBros seem to have tipped the 2016 election in favor of trump.
Now, would the same thing happen in 2020? Would approximately 1 in 10 BernieBros jump over to Trump again if Sanders loses? Who knows. After 4 years of Trump,
maybe they have learned their lesson.... holding your nose and voting for a democrat is better than the dumpster-fire that is the Trump administration. But who knows? If it ends up being a brokered convention, or Bloomberg ends up winning, maybe the same pattern will repeat itself... enough Sanders supporters voting for Trump to swing a few key states.