I have a different take on the current situation. I think the violence we are seeing is from sore losers. With the passage of health care "reform," the radical right has suffered a crushing defeat, and all they can do now is flail.
That's the good part. The bad part is: this is the authoritarian interregnum. The violence began in the 1990s, reached a peak with the Oklahoma City bombing, retreated. A questionably-decided election in 2000 put a radical-right administration (Bush II) in power, and the fear generated by an act of terror (9/11), gave the Bush II administration the mandate to do as it wished. There were numerous abuses and acts of violence perpetrated by the Bush II administration. The greatest, undoubtedly, was the Iraq war.
The Republicans were roundly defeated in 2008, and a Democratic Senate, House, and President moved in. And what have the Democrats done with their new power? Continued the Bush II torture agenda, resisted bringing the perpetrators of the crimes of the Bush II administration to justice, let the health insurance companies reform health care to their benefit, dealt women's rights the biggest blow in decades, failed to restrain the abuses of the banking system. The Democrats have become a ruling conservative party, though with a liberal wing, and they have no effective opposition. Meanwhile the Republicans have shuffled off to gamma quadrant.
We desperately need a revived left, and I think one is in the works. But it is going to take time, and another decade of corporatist government is going to be hard indeed.