If there are more employees in a union, the union is likely going to be more powerful.  There's strength in numbers.  That's the purpose of a union!  The main point is that there is nothing an employer can do to take advantage of workers.  However, if only a few workers are unionized the employer can ignore them.

Management is often very leery of unionization efforts. In fact, historically there have been many bitter (and sometimes violent) confrontations between labor and management, especially when unions are being formed in new locations. Management knows that a successful unionization effort is going to redistribute management's power to the worker, and nobody wants to give up power in th workplace. So a business that is not already highly unionized will probably not look at such efforts kindly.

I would guess that it would.  If the employer already has a group of employees in one part of the firm that is unionized, the employer will already know the pros and cons of dealing with unionized employees.  Therefore, the employer will have a different point of view than someone who was going based solely on stereotyped views of unions and labor relations.