Not quite...
I'd make the default answer to be "no" and so the number of Yes votes would have to be more than half of thiose balloted. As long as this is perfectly clear upfront, I think you'd get a sufficiently large mandate either way.
I don't believe that anyone should be sacked merely for union membership and any associated, legal, activity.
By the same token I don't agree with Union boses calling for strike action on days which are controversial. This is unnecessarily confrontational and gets little/no public sympathy...