On the day President John F. Kennedy was assassinated, the Dallas Morning News ran a giant black-bordered ad headlined "Welcome Mr. Kennedy to Dallas" that ripped him as soft on communism. Plastered around town were black-framed posters of Kennedy's face under the words, "Wanted for Treason."

As Kennedy landed in Dallas, he ominously told his wife, Jacqueline, "We're heading into nut country."

In 1963, "nut country" - both left- and right-wing - was confined to pockets like Dallas around the country. Now, extreme speech like the kind heard in Dallas 50 years ago has become mainstream - particularly on the right, amplified by social media and encouraged by gridlocked political leaders looking to win the political minute....