What Would It Take For Trump To Get Primaried?

… Is the president in danger of a serious challenge for the 2020 Republican presidential nomination? Right now, I don’t think Trump has too much to worry about. But there are two scenarios in which a primary challenger against Trump would be more viable than they are now — and either or both of them could happen in 2019. CONT.

Perry Bacon Jr., FiveThirtyEight