Political conversations have grown increasingly nasty in recent years, and it may be spawning something even worse: a growing number of people taking their vitriol to another level by threatening members of Congress. In the past couple of months, the Justice Department announced action in four cases involving people who threatened serious harm, or even death, to federal lawmakers. Most of those threats were made online, reflecting just one way people can convey their most vulgar feelings toward those on other sides of public policy disputes. Most of the targets have been Republican lawmakers, who are in the crosshairs of...