The governors of 24 states – representing more than half of the U.S. population – have signed a joint letter opposing President Donald Trump’s efforts to roll back Obama-era fuel economy rules. The list includes traditional environmentally friendly states such as California and Vermont, but opposition to the regulatory rollback has more recently garnered support from Pennsylvania, Montana, North Carolina, and Virginia.

“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 write.

California is leading the effort as the state has historically had a waiver from the U.S. Environmental Protection Agency allowing it to set its own more stringent standards. Fourteen other states have adopted California’s standards, effectively creating two sets of fuel economy rules in the U.S. The Obama-era regulations calling for 54.5mpg fuel economy by 2025 would have unified national and California standards.

Trump’s plan would freeze standards at current levels and cancel California’s waiver, also creating one national standard. Several states have sued to block that action. While the administration moves forward with its plan, lawsuits promise to drag the process out for years, likely leaving the two-tiered system in place. Automakers have argued that the Obama-era standards are too high, but they also oppose scrapping that framework entirely because of the threat of an uncertain regulatory environment and the risk of multiple standards.

“Automakers have said many times that we support year-over-year increases in fuel economy standards that align with marketplace realities, and we support one national program as the best path to preserve good auto jobs and keep new vehicles affordable for more Americans,” officials with the Alliance of Automobile Manufacturers trade group say. “It is untenable to face a marketplace with different standards in different states, but it is also untenable to face standards that rise so high that only a handful of electric vehicles can achieve them.”

Automakers have urged negotiation and compromise to create a unified standard, with little apparent progress.

In their letter, the governors call for standards that:

  • Continuously reduce greenhouse gas emissions
  • Provide regulatory certainty for the industry
  • Preserve jobs
  • Keep new vehicles affordable

Administration officials continue to defend the plan, saying lowering standards could improve fleet fuel economy by keeping costs down, allowing more Americans to replace older vehicles with cleaner new ones.

“We know that newer cars are safer and cleaner than older cars. We also know that consumers can choose whether to keep their older car or to purchase a newer, safer, cleaner car,” says Heidi King, deputy administrator of the National Highway Traffic Safety Administration. “Standards that increase the price of a new car can hinder safety by discouraging people from replacing their older cars.”;;