Eugene Robinson paints with too broad a brush. Many of the ultra far right are legitimate or playing militia and are harmless, beer drinking dolts. Further their actions and speech are protected under the constitution.
On the other hand their are racist skinheads and real wackadoos that inhabit the ultra-fringe. And they hide under the banner of the flag. I’m not sure where to draw the line, but like the anarchist who killed people in Seattle or the Earth First wackadoos, these ultra fringe groups need to be watched and monitored (subject to the boundaries of the Constitution). They are enemies within.
What about Fort Hood - terrorism from the left. Lackawanna 6, Fort Dix terrorists, 1993 World Trade Center - all terror from the left.