They would rise up in righteous indignation that any one would think it untoward for a lefty spy to be diddling the NRA just as they rise up in indignation at the terrible contravention of their Government-given Rights upon learning of the infiltration of their own org. They might even call for a law mandating Liberal officers from Left orgs be placed in such organizations as NRA as a requirement for their continued existence.
It wouldn't surprise me if there already was one.....