Ajamu 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. Earlier this year, the former executive director of the U.S. Human Rights Network had his work featured in a book edited by accused Holocaust denier (“the Holocaust controversy was a legitimate topic of historical debate”) and self-described 9/11 truther Kevin Barrett. The book, Another...