DETROIT — The Trump administration is expected to announce that it will roll back automobile gas mileage and pollution standards that were a pillar in the Obama administration's plans to combat climate change.

It's not clear whether the announcement will include a specific number, but current regulations from the Environmental Protection Agency require the fleet of new vehicles to get 36 miles per gallon in real-world driving by 2025. That's about 10 mpg over the existing standard.

Environmental groups, who predict increased greenhouse gas emissions and more gasoline consumption if the standards are relaxed, say the announcement could come Tuesday at a Virginia car dealership. EPA spokeswoman Liz Bowman said in an email Friday that the standards are still being reviewed. 

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. About a dozen other states follow California's rules, and together they account for more than one-third of the vehicles sold in the US. Currently the federal and California standards are the same. 

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 due to the cost of developing technology needed to raise mileage.

But environmental groups say people will save money at the pump, and the technology is available for the industry to comply.

They also say burning more gasoline will put people's health at risk.

"The American public overwhelmingly supports strong vehicle standards because they cut the cost of driving, reduce air pollution, and combat climate change," said Luke Tonachel, director of the Natural Resources Defense Council's Clean Vehicles and Fuels Project.

The EPA and the National Highway Traffic Safety Administration are involved in setting the standards, which would cover the years 2022 through 2025. 

Some conservative groups are pressing EPA Administrator Scott Pruitt to revoke a waiver that allows California to set its own rules. They say California shouldn't be allowed to set policy for the rest of the nation. Pruitt has publicly questioned the veracity of evidence complied by climate scientists, including those in his own agency, that global warming is overwhelmingly caused by man-made carbon emissions from burning fossil fuels.

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."

Getting rid of the waiver or having two different gas mileage and pollution requirements presents a huge dilemma for automakers: while they would like to avoid fines for failing to meet the standards, they also don't want the expense of building two versions of cars and trucks, one for the California-led states and another for the rest of the country.

Mark Reuss, a General Motors executive vice president who heads product development, said in a recent interview that he would rather have a single nationwide standard, even if it stays the same. He called two standards "just waste," because they would require different equipment on vehicles and costly additional engineering. "I want one good one. That would be wonderful," he said. "I could focus all my engineers on one." 

Automakers agreed to the fuel economy standards in 2012 during the Obama administration, but lobbied for and received a midterm review in 2018 to account for changes in market conditions. In the waning days of Obama's presidency, the EPA did the review and proclaimed that the standards have enough flexibility and the technology is available to meet them. 

It's likely to take years for the matter to be resolved, and it could become an issue in the 2020 presidential election.

Janet McCabe, who was acting assistant EPA administrator under Obama when the review was done, said Friday it will take a couple years for the EPA to propose new rules, gather public comment and finalize any changes. Any rollback would likely bring legal challenges, forcing Pruitt's EPA to defend the science behind the changes.

"This would all take a long time," said McCabe, now a senior fellow at the Environmental Law and Policy Center. 

In the meantime, automakers have to proceed with plans for new vehicles under the current gas mileage requirements because it takes years to develop vehicles.