I am a labor attorney who is a former National Labor Relations Board attorney and former Director of Labor Relations for a major company and currently teach Labor Relations in an MBA program.
Under the current law, a union can petition the Naitonal Labor Relations Board to conduct a secret ballot election if it has a showing of interest, usually signed cards, from 30% of the employees. The NLRB conducts the election under pristine secret ballot methods. the proposed law would allow for a second process for the NLRB to certify a union, through showing that the union has over half of the employees signing to indicate their membership in the union. The "card check" is a key part of the proposed legislation. That said, there is now a lot of speculation that the proposed law will probably be amended to not have this "card check" provision but to rather require the NLRB to conduct the secret ballot elections in a much quicker process, maybe within days of a petition being filed.
Please post any other questions on this matter and I would be happy to offer responses. I will not take sides on this issue.