Many construction contractors work with firms such as Tradesmen International to staff projects, says Geoff Burr, the Associated Builders and Contractors' vice president of government affairs. He says, "Those construction companies could be deemed to be responsible for the behavior of workers at those staffing agencies ... so this rule could have an impact on those sorts of relationships."

Burr adds that the decision also could affect certain contractor-subcontractor relationships, if contractors are responsible for employees of a subcontractor that it doesn't control.

Daniel Johns, a partner with law firm Ballard Spahr, says the decision may confuse the issue of which jobsite gate is the proper place for employee picketing.

Currently, if workers have a dispute with a subcontractor, they can picket at a specially reserved jobsite gate. A separate gate for the owner and general contractor enables work to continue. But Johns says, "If some of those other firms are considered joint employers, your reserved-gate system doesn't work."

The NLRB's dissenting board members chided the majority. "The board is not Congress," they wrote.

AFL-CIO President Richard Trumka in a statement noted, "Labor laws in America have failed to keep pace as the workplace has continued to evolve."