Margaret Sullivan: Beto, Biden and Bernie: The B-Boys and the media’s dangerous, self-fulfilling prophecy

Former Texas congressman Beto O'Rourke gestures during a campaign stop at Keene State College in Keene, N.H., Tuesday, March 19, 2019. O'Rourke announced last week that he'll seek the 2020 Democratic presidential nomination. (AP Photo/Charles Krupa)