Sarah Palin's campaign from earlier this year. I would have thought before that it's just coincidence, and I still lean that way. But after the guy who admired Glenn Beck went and shot up an office of an organization that Beck had railed on, it caught my attention. Not only that, but there was also the FBI thing about fertile ground for right-wing militias and extremists. The left has it's fair share too, environmentalists, animal rights activists, I think maybe that some people have to start being more careful how ramped up they get with their rhetoric. Even if it doesn't encourage nutbars like this @sshole, milder discourse would be better.
The Palin campaign, using crosshairs on a US map to target House Democrats who voted for the health care reform package: