A federal government ruling yesterday broadening the definition of supervisory workers could limit the ability of millions of skilled employees to join unions, possibly accelerating the decades-long decline in union membership.

In a long-awaited decision, the Republican-controlled National Labor Relations Board in effect classified many nurses as supervisors if they direct and oversee other nursing staff.

Under federal law, employees defined as supervisors aren't entitled to legal protections ensuring their right to join unions.

The NLRB definitions also could be applied to other workers, particularly in fast-growing service-sector jobs where unions have made some gains in recent years even as overall union membership has declined nationally, labor experts said.

Many service-sector employees, such as those working in restaurants or retailers, have been classified as "shift supervisors" even though most of their duties — such as waiting on tables or cashiering — are indistinguishable from those they supervise.

Several large retailers, including Home Depot Inc., Abercrombie & Fitch Co. and Staples, have been targets of lawsuits in recent years by employees claiming they were denied overtime pay because they were classified as supervisors, even though only a fraction of their responsibilities involved supervision.

The labor board's ruling was applauded by business organizations but denounced by labor groups, who called it part of a Bush administration strategy to "destroy" unions.

"It's an outrageous decision," said Rose Ann DeMoro, executive director of the 70,000-member California Nurses Association, which threatened to strike against hospitals that try to enforce the new rule.

AFL-CIO President John Sweeney said the labor board's decision will immediately deny as many as 8 million workers "their basic right to have a voice on the job."

The ruling is likely to affect primarily work sites where organizing is going on, said Stephen A. Bokat, general counsel at the U.S. Chamber of Commerce. "Where there are established collective bargaining relationships," he said, "these issues with regard to supervisors are very well established and I doubt most employers will totally upset their workplaces to meet that definition."

Federal law defines supervisors as those whose jobs may require them to use "independent judgment" in the course of their duties, or have ongoing responsibilities to "assign" or "responsibly direct" the activities of other employees.

Yesterday's ruling opened the door to categorizing more employees as supervisors. For instance, the labor board determined that the term "assign" could include dispatching an employee to a work station, appointing an employee to a specific shift or to work overtime, or directing a nurse to care for a particular patient.

The five-member NLRB actually issued rulings in three related cases yesterday. In the first, it voted 3-2 that permanent charge nurses at a Michigan hospital routinely perform such supervisory duties, but rejected that designation for charge nurses who rotate in and out of that job. The board's three Republicans voted in favor of the ruling, with the two Democrats opposing it.

In the two other cases, the board found that employers did not provide enough evidence to determine whether charge nurses at a Minnesota nursing home or so-called lead workers at a Mississippi manufacturing plant should also be classified as supervisors.