Militant movements tend to do best when the powerful and well-connected are giving them weapons and legal cover. This is as true in America as anywhere else. In order to have a surge of right-wing terrorism, you need to have at least one powerful section of society or government who, if not exactly sympathetic with their values, at least wants to use them as tools to further their domestic power. Otherwise, you'll just see a replay of the 90's: a minor uptick in violent activity, followed by a fairly effective crackdown.
So off the top of my head, here are a few quick scenarios.
- A "deep state" alliance between the FBI and pentagon forms, decides that the current order isn't to their liking for whatever reason, and starts using militias as proxy forces.
- A second "businessman's plot" forms, this time a bit more cautiously and smartly. Using their connections to the halls of power, they avoid prosecution while secretly supporting the militia movement as their own blackshirts.
- Local bigwigs, feeling threatened by the power of corporate titans and the feds, begin to quietly shift money and resources to their favored proxies. Think along the lines of a violent Tea Party movement.
The challenge then is to find a POD where something like this is feasible.