Two dozen governors from across the country, including two Republicans and governors of four states that voted for President Donald Trump in 2016, are set to urge his administration Tuesday to halt one of his biggest climate policy rollbacks: the weakening of federal clean car rules.

The governors will also call on Trump to honor California’s legal right to write its own clean air rules, something the administration has said it is set to challenge, according to a draft statement by the governors.

“Strong vehicle standards protect our communities from unnecessary air pollution and fuel costs, and they address the largest source of carbon pollution in the United States,” the governors wrote. “We must unite to ensure a strong, science-based national standard, in California and across the country, that increases year over year.”

Even automakers have balked at the Trump administration’s plan, which in its most extreme outline proposes to substantially weaken Obama-era standards that would have doubled the fuel economy requirement of new cars, pickup trucks and sport utility vehicles by 2025.

Last month, 17 automakers asked Trump to soften his approach, saying his plan threatened to hurt their profits and produce “untenable” instability given that California — as well as 13 other states that follow its lead — is expected to stick with the stricter standards. That raises the specter of a national auto market split in two, and a messy legal battle.

Now, the governors’ demands add pressure on the Trump administration to reconsider what has become a highly fraught rollback of environmental rules. According to the draft statement, its signatories represent 52% of the United States population and include two Republican governors — Charlie Baker of Massachusetts and Phil Scott of Vermont — as well as the governors of four states that voted for Trump in 2016: Montana, North Carolina, Pennsylvania and Wisconsin.

The broad call, led by Gov. Gavin Newsom of California, creates an awkward moment for Trump, who has also withdrawn the United States from the Paris climate accord and sought to weaken more than 80 environmental regulations but nevertheless talked up “America’s environmental leadership” in a speech Monday.

Assuming the Trump administration’s rollback of clean car rules is finalized and survives legal challenges, America’s cars and trucks would emit as much as an extra 321 million to 931 million metric tons of carbon dioxide into the atmosphere by 2035 as a result of the weaker rules, according to an analysis by the research firm Rhodium Group.

Mary Nichols, California’s clean air regulator, said California was already winning the battle.

“We have the largest group of states ever coming together to back our position,” she said. “The fact that we now have over half the U.S. auto market supporting us indicates that we are going to stick with the standards. The auto industry will not build two sets of cars.”

Nichols rejected any possibility of compromise on the Obama-era goals for fuel efficiency that would require automakers to put vehicles on the road averaging roughly 36 mpg by 2025, a pace some automakers had called unattainable.

However, she said, California was open to negotiating on ways to make it easier for automakers to meet those targets, including the use of more credits to reward fuel-saving technologies.

The White House did not immediately return requests for comment.

This article originally appeared in The New York Times.