How great is the threat of right wing extremism? Like KKK,TEA Parties, Militias?

What will happen when they lose the election and become even more marginalized? I'm afraid they will form groups like the Taliban or Hitlers Brown shirts. They already kill people who perform abortions and they shot that lady in AZ. Should someone be keeping tabs on these crazies. Maybe a stint in prison will cure their racism and treason.