The administration also contends the new rule would reduce "societal costs" by about $500 billion over the life of the vehicles but the administration's overall forecast net benefits are unclear, once higher fuel consumption is taken into account.
"There's no question it will be litigated at every turn, and no doubt it's going to make it to the Supreme Court", with its new mix of judges, possibly including President Donald Trump's latest nominee, Brett Kavanaugh, she added.
U.S. Environmental Protection Agency acting Administrator Andrew Wheeler said on Tuesday that the agency wanted a "50-state solution" to the nation's vehicle emissions standards and would also pursue reforms to the country's biofuel policy. The EPA is expected to propose revoking the Clean Air Act waiver given to California.
The proposal - presuming it sees the light of day - will be the first shot in what is expected to be a long battle in the USA courts.
A White House spokeswoman did not immediately respond for comment.
If Trump's plan sticks, it could be his biggest regulatory rollback yet.
Trump's move could also strip California of powers to set electric vehicle quotas within the state.
The proposal is sure to spark a massive fight with California and a dozen other states that have adopted California's emissions rules, which would require average vehicle fuel efficiency to reach around 50 miles per gallon by 2025.
And a long legal fight between the state and federal governments could make it hard for the automakers to plan, since the process of designing, engineering and introducing a new vehicle typically takes more than three years, Brauer said.
But if California and the 12 other states and the District of Columbia retain their own emission rules, auto makers would have to either market two different sets of cars or sell California-compliant cars nationwide.
In its legal analysis, Holmstead said, EPA could determine that California did not face "compelling extraordinary conditions", compared to other states, in needing to reduce greenhouse gases, such as carbon dioxide. The waiver was granted at a time when California suffered some of the worst smog problems in the country. NHTSA is planning to argue that a 1975 law that enacted the first federal fuel efficiency standards prohibits the state from regulating tailpipe emissions. California and automakers agreed to the rules in 2012, setting a single national fuel economy standard. So California's standards are now in sync with the federal government's. And yet, auto companies don't want California to once again have its own standard, if the Trump administration succeeded in rolling back federal fuel-economy standards but California was able to maintain its emissions requirements.
"It's an open question whether that provision (in the Clean Air Act) was meant to only deal with local pollution involving extraordinary conditions", said Sivas, who was an attorney for Earth Justice, an environmental law firm.
For its part, the American Trucking Associations said that although the current Trump administration moves are targeting passenger cars, it will be keeping an eye for new developments that affect the trucking industry.