I’m assuming at will employment makes this very unlikely to be successful.
Well you can't fire people for political reasons. It's a protected category. So the crux of it will likely depend on if there's similar protests from opposing viewpoints that were not dealt with similarly. Eg. If there were any counter protestors at the same event that weren't also fired that's a reasonably clear cut case of firing a group for political views.
Political affiliation is not a protected class. California law does restrict employers from punishing employees for their political activities, but this generally applies to activities done outside of work.