WASHINGTON  The Labor Department is racing to complete a new rule, strenuously opposed by President-elect Barack Obama, that would make it much harder for the government to regulate toxic substances and hazardous chemicals to which workers are exposed on the job.

The rule, which has strong support from business groups, says that in assessing the risk from a particular substance, federal agencies should gather and analyze “industry-by-industry evidence” of employees’ exposure to it during their working lives. The proposal would, in many cases, add a step to the lengthy process of developing standards to protect workers’ health.

Public health officials and labor unions said the rule would delay needed protections for workers, resulting in additional deaths and illnesses.

With the economy tumbling and American troops fighting in Iraq and Afghanistan, President Bush has promised to cooperate with Mr. Obama to make the transition “as smooth as possible.” But that has not stopped his administration from trying, in its final days, to cement in place a diverse array of new regulations.