The news that the Trump administration is expected to roll back Obama-era EPA regulations on car emissions and fuel efficiency standards is tough to swallow for Harvard Law professor Jody Freeman. She negotiated those standards with the auto industry in 2009 and 2010 as a counselor for energy and climate change in the Obama White House.

“It was kind of a historic achievement,” she said. “And it’s pretty dramatic to think that they might be rolled back now.”

The standards require automakers to achieve an average of 36 miles per gallon across their car models by 2025.  That's about 10 mpg over the standards in effect for 2018 models.

Environmental groups say the announcement of a change in the EPA standards could come Tuesday at a Virginia car dealership. EPA spokeswoman Liz Bowman said in an email Friday that the standards are still being reviewed.

Freeman said she’s not sure it’s in the best interest of car companies to unravel that deal she brokered with them. “They got one set of national standards. Whereas before there was a patchwork system,” Freeman said. “I think they really got something valuable from the deal, which was certainty and predictability and one set of targets to meet. So I actually don't think they want to see this whole thing come apart.”

And yet, some automakers have lobbied to revisit the requirements, saying they'll have trouble reaching them because people are buying bigger vehicles due to low gas prices. They say the standards will cost the industry billions of dollars and raise vehicle prices to cover the expense of developing technology needed to raise mileage.

Any change is likely to set up a lengthy legal showdown with California, which currently has the power to set its own pollution and gas mileage standards and doesn't want them to change. Massachusetts is one of about a dozen other states that follow California's rules. Together, those states account for more than one-third of the vehicles sold in the US. Currently the federal and California standards are the same.

“So the auto industry could be facing both a federal set of targets and then this state level set of targets led by California,” Freeman said. “And that just doesn't work.”

The Trump administration could move to revoke the federal waiver that allows California to have stricter standards.

“If it turns out that the EPA revokes the California special authority, the waiver, that means all the states that follow California won't be allowed to do the same thing,” Freeman said. “And that's very bad news for all of the states that have kind of moved toward cleaner air and move toward greenhouse gas pollution control."

If the waiver is revoked, California Attorney General Xavier Becerra says the state will resist. "What we're doing to protect California's environment isn't just good for our communities — it's good for the country," he said in a statement. "We're not looking to pick a fight with the Trump administration, but when they threaten our values, we're ready."

Freeman said Massachusetts would likely join California in suing the federal government over the rollback. “This is not what Massachusetts voters and residents have been supporting,” she said. “They want to go in a different direction. So all of this sort of regulatory rollback is not in the interests of the residents of the state.”

Attorney General Maura Healey's office has not yet commented on a potential suit.

The federal government is still compelled by a Supreme Court ruling to regulate carbon dioxide emissions, said Robert N. Stavins, a professor of energy and economic development at Harvard’s John F. Kennedy School of Government. But Stavins said the Trump administration will likely redo the “regulatory impact analysis” done by the Obama administration, which looks at the cost and benefits of regulation.

 “They will re-do that as they’ve done for other regulations and come up with their own calculations, which presumably point to much less ambitious standards as being efficient,” Stavins said.

The standards require the automakers to hit an average efficiency level across all their models. “So people in Massachusetts are still going to be able to buy a Prius,” Stavins said. But the change in standards would allow manufacturers to sell more low-efficient models.

“The result of this, environmentally, is that because the mix of cars that are sold in any year in the future will be less fuel efficient than they otherwise would have been under the Obama standards, it means that people as a whole the United States will be using more gasoline, that will be costing them more, in terms of the gasoline purchases, and it also means there will be more carbon dioxide emissions, which come from the combustion of gasoline and diesel fuel,” Stavins said.

The environmental impact wouldn’t end there, Stavins said. “Not only do automobiles emit carbon dioxide they emit a whole set of other pollutants that are correlated with those CO2 emissions.” Those include air pollutants such as nitrogen oxide, sulfur dioxide and particulates.

The Associated Press contributed to this report.