Should Sanders prevail, Trump's strategy will be to spotlight his democratic-socialist identity in an attempt to make voters fear he’ll take away their freedom. (Trump will try to brand any Democratic nominee a socialist—Team Trump just thinks it'll be easier with the guy whose self-description includes the word.) If Sanders falters, Trump will argue that he was unfairly robbed of a nomination he earned. Trump has long stoked suspicions of an anti-Sanders conspiracy within the Democratic Party, for what seems to be two purposes: leaving Sanders’s following so disillusioned that they stay home on Election Day, or perhaps persuading them to switch sides and vote Republican. Sanders may not need Trump, but for the time being, Trump needs Sanders.
Seems like the best case scenario for Trump would be a contentious primary that ultimately goes to Biden. Or...say...a contested convention that makes progressives angry at the party!