Google aspires “to organize the world’s information and make it universally accessible and useful.” Facebook’s mission is to “make the world more open and connected.” Twitter, while occasionally billing itself as “the free speech wing of the free speech party,” prohibits “hateful conduct,” telling users: “You may not promote violence against or directly attack or threaten other people on the basis of race, ethnicity, national origin, sexual orientation, gender, gender identity, religious affiliation, age, disability, or disease.”

The radical right was more successful in entering the political mainstream last year than in half a century. How did it happen?