As with Biden, Trump wouldn’t be able to rely on his favorite identity politics gambit against Bernie Sanders. So what would his campaign look like?
This one is obvious: socialism and Sandinistas. Trump would portray Sanders as a threat to economic prosperity and property rights at home, and a wimp with dangerous leftist leanings abroad.
A large majority of Americans (older voters in particular) are terrified of socialism, so that part of the argument would stick. I’m not sure that pictures of Bernie with the Castros and Daniel Ortega will resonate with the electorate (at least outside of Florida) in 2020, however. Those days may just be too remote to matter.
Could Sanders beat Trump? Barring a fairly severe economic downturn between now and November, probably not. He would have a better chance than Warren, however, even though she would make a much better president.