It seems relatively simple. The proposed federal Employee Free Choice Act would give employees the freedom to form a union when a majority of workers sign cards saying that they want one, avoiding the often months of employer harassment that have inevitably accompanied traditional National Labor Relations Board (NLRB) election processes.
State laws allowing majority sign-up for groups of public and private employees have been enacted in California, Connecticut, Illinois, Iowa, Kansas, Maryland, Massachusetts, Michigan, Minnesota, Montana, New Hampshire, Nevada, New Jersey, New York, North Dakota, Ohio, Oklahoma, Oregon, Washington, and Wisconsin. Examples of these statutes include Oregon Revised Statutes, 243.682 for public employees and New York Chapter 31, Article 20, Section 705 covering both public employees and a number of private industries.
So you’ve got one system — NLRB elections with a demonstrated history of massive, overwhelming employer abuse — and another system — majority signup operating in many states with no evidence of any of the abuses alleged by opponents. If it works in the states, why not bring its benefits to more employees?