I don't think these right wing militias are that much of a threat. They're mostly LARPers and have enough attachment to their groups to not do the radical violence. Historically the right wingers that have actually committed violence are the ones that can't even find belonging in the right wing militias.
They're essentially the US' guerilla forces that we see in other countries that cause foreign forces operating in their land to get stuck in a quagmire. They may not be undefeatable, but they'll be able to put up a helluva show enough to make the opfor to question their commitment. Probably long enough to sway public opinion as well.
