You would need an economic downturn, a broad loss of faith in political institutions, a sitting president very unpopular with their own party, and an opposition party with lackluster options of whom no one really stood out, and a popular figure with a lot of name recognition either from the business world or defecting from a major party to pick up the support. It would have to be a mix of what happened in 1920, 1968, 1992, and 2016.