In a nutshell, here is how Trump and the GOP will attack the leading candidates for the Democratic nomination in 2020. Please note that I am simply laying out lines of argument, some of which will be handled as dog whistles, and others made explicit; I am not in any way endorsing or justifying them.
SANDERS: Socialism! Sandinistas! Venezuela! Jewish radical! (There will actually be some truth to these statements)
WARREN: Socialism! Pocahontas! Snooty Ivy League professor who talks down to you and despises your values! Uppity woman telling men what to do!
BOOKER: Socialism! Patronizing Ivy League egghead who doesn’t understand your real American values! Newark is a crime-infested hellhole populated mostly by those people!
HARRIS: Socialism! Soft on crime and illegal immigration! Uppity woman telling men what to do! Low morals! San Francisco liberal who hates your values and loves those people!
BIDEN: Plagiarism! Obama connection!
If you read this carefully, you can see that it is an argument for Biden as the party nominee. He’s simply not as vulnerable to GOP attacks as the others. His real weaknesses are with his own party; they won’t show up standing next to Trump.