Traffic is congested in Bethesda during rush hour in October 2016. (Sarah L. Voisin/The Washington Post)

Nearly two dozen governors, including several Republicans, are imploring the Trump administration to embrace more aggressive gas mileage requirements for the nation’s cars and pickup trucks, saying the White House’s effort to weaken existing requirements could hurt consumers, automakers and the environment.

In a joint statement Tuesday, governors from 22 states and Puerto Rico joined California in calling for a “common sense” national approach that would provide regulatory certainty to the auto industry while also helping to combat climate change. “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,” they wrote.

Mary Nichols, chair of the California Air Resources Board (CARB), said Tuesday’s statement underscores that a growing number of states are willing to back California in its push to cut carbon emissions and other pollution from the nation’s tailpipes. Currently, 13 states and the District of Columbia have said they are poised to adopt California’s standards if they diverge from the federal government’s.

“We’ve doubled our support,” Nichols said in an interview. “We’re seeing an awakening of interest on the part of other states that recognize the need for cleaner cars. This is not just about climate change, although that is certainly the major thrust of the regulations we’re fighting to maintain."

The group said it represents 52 percent of the U.S. population and 57 percent of the economy. It includes primarily Democratic governors, but also at least three Republican governors — from Maryland, Massachusetts and Vermont. Gretchen Whitmer, the Democratic governor of Michigan — the epicenter of the U.S. auto industry — did not sign Tuesday’s statement. But a spokeswoman said Whitmer “is a proud member of the U.S. Climate Alliance” and remains “committed to reducing greenhouse gas emissions consistent with the goals of the Paris agreement.”

Their letter comes as the Trump administration is expected this summer to freeze fuel-efficiency requirements for six years and end California’s long-standing authority to set its own standards. That looming fight has thrust uncertainty into the auto market and raised the prospect of a drawn-out legal battle between federal officials and the nation’s most populous state.

In coming weeks, the Environmental Protection Agency and the Transportation Department are likely to finalize a proposal to set federal car standards at roughly 37 miles per gallon, rather than raising them to nearly 51 miles per gallon for 2025 models — the level that states, automakers and the federal government agreed to during the Obama administration. The revised rule also would revoke California’s existing waiver to set its own rules under the Clean Air Act, a practice the federal government has permitted for decades.

California leaders have shown no sign of backing down, saying they would sue over any effort to scale back the state’s autonomy and insisting the state would continue to move forward with more stringent emissions standards.

The Trump administration has remained steadfast in its plans to scale back the existing efficiency standards, arguing that the Obama-era requirements would make vehicles more expensive and encourage people to stick to driving older, less safe cars and trucks.

“As the administration has long stated, we are pursuing one national standard to provide safe, affordable vehicles for consumers while also improving environmental outcomes,” EPA spokesman Michael Abboud said in an email Tuesday.

Last month, in a last-minute push, 17 U.S. and foreign firms wrote to both President Trump and California Gov. Gavin Newsom (D), urging them to “resurrect” talks over creating a single set of fuel standards. They warned that only a nationally agreed-upon set of rules would avert “an extended period of litigation and instability, which could prove as untenable as the current program.”

But the White House quickly dismissed the automakers’ request, making clear it saw little hope of further negotiations with CARB, which it said at the time “failed to put forward a productive alternative” to the administration’s proposal. CARB officials have said the administration broke off communications late last year without any serious discussions of a compromise.

Canada also publicly sided with California recently, signing an agreement with the state to advance the development of cleaner vehicles and fuels. The deal does not require Canada to meet California’s vehicle emissions standards, but it does align the country with a state that has been consistently at odds with the Trump administration over its environmental policies.