You can delegitimize all political speech you dislike by suggesting that it may inflame the violence-prone. The press attempts this again and again. Rather than openly debate, they smear. The 1994 election, which unseated the Democratic majority, was described by the press, without any evidence, as the eruption of "angry, white, male" voters. One fan out of 8,000 at a McCain/Palin rally was reported to have shouted "Kill him" in reference to Obama. Rafts of stories dwelt upon this revelation of the "ugly" side of Palinmania. A later investigation found no evidence that it had even happened. And of course the tea party movement, a spontaneous, widespread upwelling of grassroots dismay at the direction of government policy, has been falsely and savagely maligned as racist, violent and primitive. (The best poster spotted at a tea party rally: "It doesn't matter what I put on my sign because you will accuse me of racism anyway.")
Actually, Democrats are reduced to warning that certain attitudes can lead to violence because there hasn't been any actual violence at the tea party rallies. All have been remarkably orderly and even friendly. You can almost feel the Democrats' frustration at this.
By contrast, many, many left-wing protests and demonstrations have sparked violence. Just last year, at the G-20 meeting in Pittsburgh, rampaging protestors broke shop windows and scuffled with police, who used batons and tear gas to subdue them. A 1999 meeting of the World Trade Organization in Seattle was so badly disrupted by anti-globalization fanatics, who smashed windows and shut down the center of the city, that the governor had to declare a state of emergency and call out the National Guard. (President Clinton failed to assail those who criticize corporations as inspiring the violence.)
In 2007, several hundred protesters who descended on Washington, D.C., during the International Monetary Fund meeting turned over trash cans, smashed windows, threw bricks, and pushed a police officer off her motorcycle.
In 2008, as John Hinderaker of Powerlineblog recalls, anti-Republican protesters at the convention in St. Paul: "threw bricks through the windows of buses, sending elderly convention delegates to the hospital. They dropped bags of sand off highway overpasses onto vehicles below." The violence was only fleetingly covered in the press and went unmentioned by leading Democrats.
Republicans have been very quick to condemn violent acts or even intemperate words by right-wing individuals or groups. They've even condemned some that didn't happen -- like false account of racial slurs shouted at members of the Congressional Black Caucus. While it's important to police one's ranks, it's also necessary to expose the Democrats' persistent and malignant libels.