Republicans are going to play every trick in the book, legal, moral, ethical or not. They will shout it at the rallies, take every opportunity to stir up shit on television (with help from Koch and Sinclair owned media) and you will hear every reason why the democratic nominee cannot be President. If Democrats find someone with a squeaky clean record and the Republicans can’t find anything truthful, they’ll fabricate it with ease. “Kamala Harris ate a moldy bagel in 2018 therefore she can’t be president.”
The one advantage Democrats have at this very second, is that Republicans and Trump can’t use the media machine to pre-emptively smear any one person, since nobody knows who the nominee will be yet. Use this opportunity well and combat the “ahh confusion, somehow only Trump makes sense” narrative that will be sure to be floated.