Four billboard trucks bearing the message “Stop the Liberal Bias, Tell the Truth!” began circling the Manhattan headquarters of ABC, CBS, NBC, and the New York Times on Friday. The trucks will do so for eight hours every weekday for the next four weeks as part of a campaign run by the Media Research Center, a watchdog group that analyzes the media for liberal bias. Similar trucks also are operating in the Washington, D.C. metropolitan area, passing the offices of the broadcast networks, the Washington Post, CNN, the Newseum, the National Press Club and Politico, and ads about the campaign...