Yes the forces can be invited or imposed. Real governors would handle it, as we have seen some don’t. During Rodney King riots, the governor requested the assistance.
The left fears an early call for Trump will shut down turnout in the West. If that happens, GOP will pick up house seats. Also, early sign of loss will bring out the mayhem and further shut down urban voting.
The left knew this and has done all it could to turn out their vote early.
And since the places most likely to have serious rioting are the places most likely not to request.... Imho if it’s widespread and bad enough, POTUS will invoke it. Like if people are being killed, and very widespread. Worse than what’s happened already of course.