From George Packer, who notes (correctly) that he called out the left for its ugly rhetoric in the runup to the Iraq War, on where today's ugly rhetoric mostly comes from:
In fact, there is no balance—none whatsoever. Only one side has made the rhetoric of armed revolt against an oppressive tyranny the guiding spirit of its grassroots movement and its midterm campaign. Only one side routinely invokes the Second Amendment as a form of swagger and intimidation, not-so-coyly conflating rights with threats. Only one side’s activists bring guns to democratic political gatherings. Only one side has a popular national TV host who uses his platform to indoctrinate viewers in the conviction that the President is an alien, totalitarian menace to the country. Only one side fills the AM waves with rage and incendiary falsehoods. Only one side has an iconic leader, with a devoted grassroots following, who can’t stop using violent imagery and dividing her countrymen into us and them, real and fake. Any sentient American knows which side that is; to argue otherwise is disingenuous.
This is too obviously true to need much defense. I don't really blame conservatives for being upset at liberals trying pin the blame for the Giffords shooting on them, but the furious defensiveness of their counterattack says all that needs to be said about how uncomfortable they are with their own recent history. The big difference between right and left, as I and others have noted repeatedly, isn't just in the amount of violent rhetoric, but its source. On the liberal side, it only occasionally comes from movement leaders. On the right, it regularly does. It comes from opinion leaders, political leaders, and media leaders, and the more heated they get, the more popular they get. As David Corn says, "Republicans have institutionalized their side's craziness." This is the big difference between the two sides, and the right could really stand to engage in a wee bit of soul searching over this.