Led by California Gov. Gavin Newsom, governors from 23 states released a joint statement urging Trump to reconsider the rollback of environmental regulations on cars, trucks and SUVs. The list includes the Republican governors of Massachusetts and Vermont.
The statement calls for a "common sense approach."
The Obama administration standards would require vehicles averaging 36 miles per gallon by 2025, something some automakers say isn't possible.
But environmentalist warns that the proposal to freeze emission standards at 2020 levels would emit an extra 321 million to 931 million metric tons of carbon dioxide into the atmosphere by 2035.
"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 governor's wrote. "We must unite to ensure a strong, science-based national standard, in California and across the country, that increases year over year."
The battle is especially important in California where the state has its own environmental regulations for vehicle emissions. Last month, 17 automakers joined the fight to keep the current standards followed by California and other states.
California Air Resources Board chairman Mary Nichols said it doesn't make sense for the auto industry to build two sets of vehicles.
"We have the largest group of states ever coming together to back our position," Nichols told the New York Times. "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."