The U.S. Supreme Court on Monday will weigh efforts by President Barack Obama's administration to regulate greenhouse gases, which industry groups and Republican-led states are labeling gross overreach.
Experts say even if the court sides with businesses, it would barely dent the administration's efforts to tackle global warming.
But businesses worry they could face huge additional costs if the Environmental Protection Agency wins, as it could then order industrial sites to be designed and operated in specific ways that differ from current practices.
Frustrated by inaction in Congress, the EPA adopted regulations in 2010 to limit carbon emissions by stationary facilities, such as power plants, and by motor vehicles.
The court announced in October that it would review a petition by the National Association of Manufacturers and other groups challenging the EPA's greenhouse gas regulations from stationary sources, declining to take up other appeals questioning the agency's broader authority to fight global warming.
Under the rules, new power plants, factories and other stationary industrial sites would have to use the latest energy-efficient technologies.
But industry groups have criticized the regulations as bad for the economy.
NAM President and CEO Jay Timmons said the regulations were "one of the most costly, complex and harmful regulatory issues facing manufacturers and threatening our global competitiveness."
A group of 12 Republican-led states, including Texas and Michigan, called it "one of the most brazen power grabs attempted by an administrative agency."
Environmental activists and judges stress that the case is very narrow in focus.
"It's yet another attempt to weaken EPA legal authority to regulate greenhouse gases... but it does not go to the heart of EPA authority," said Elliot Diringer, executive vice president of the Center for Climate and Energy Solutions in Washington.
"The outcome of this case is very unlikely to derail EPA efforts to limit carbon emissions from power plants or vehicles," he added, noting that the worst possible outcome for the agency would only be a delay in its requirements for energy-efficient technologies.
Harvard Law School professor Jody Freeman noted that "even if the government loses, the impact on the EPA's regulatory agenda for greenhouse gases, and thus on the president's climate action plan, is fairly small and entirely manageable."
To create the regulations, the EPA relied on provisions in the Clean Air Act of 1970 it said suggest that issuing standards on vehicle emissions meant it could also adopt rules for stationary sources of greenhouse gases.
At first, the agency proposed stricter regulations on carbon dioxide emissions for coal electric plants.
In September 2013, the agency then proposed for the first time limits on the carbon emissions of existing electric plants.
But the measures targeting electric plants, along with those aimed at vehicle emissions, are not affected by the Supreme Court decision in the current case.
"The largest sources of US emissions, vehicles and (electric) plants, are not directly affected by this case," Diringer told AFP.