I think they have been successful lately because the president is a black man.
Maybe, but this is not unprecedented. Remember the big surge of "militia", religious, and secessionist nuttery back in the 90's when Bill Clinton was president? Ruby Ridge, Branch Davidians, OKC bombing, Republic of Texas, etc.? I suspect that any Democratic president would have sufficed, black or not. Hell, even George W. Bush was too far left for some of these nuts.