https://www.youtube.com/watch?v=NF4DR8dTEuAAjamu Baraka, professorial with kind eyes, was an accomplished human-rights activist before joining the Green Party ticket as Jill Stein’s running mate.

But the two leftist candidates—due for a mainstream close-up in a CNN town hall on Wednesday—also have engaged with some fringe thinkers and conspiracy theories. Baraka, especially, has an accomplished résumé in this field....