To my understanding if 50% (a majority) of the employees signs up (ie, union cards) for the union prior to any vote, if the employer takes a union-busting act, the union is automatically legitimized and the employer must negotiate with them.

Most major corporations are all already doing union-deterrence (not illegal) as operating practice so they likely won't see an impact unless there is a black swan event that causes workforce and management to come to loggerheads.