WASHINGTON – Citing climate-damaging tailpipe emissions, 23 U.S. governors, including Colorado’s Gov. Jared Polis, signed a pledge backing California leaders in their showdown with the Trump administration over its plans to relax vehicle mileage standards.

The stand by leaders of states and Puerto Rico, nearly all Democrats, comes as the Trump administration moves to freeze tougher mileage standards laid out by former President Barack Obama, in one of the previous administration’s key efforts against climate change.

The Trump administration says American consumers increasingly want bigger, gas-guzzling SUVs and pickup trucks. It also argues that demanding ever-more fuel-efficient vehicles will drive up automobile costs and keep less-safe, older vehicles on the road longer. Many engineers have challenged that claim.

The governors’ pledge on Tuesday commits to sticking to the pre-Trump mileage goals, a program of annual tightening in mileage standards that reduce climate-changing carbon emissions.

“We will not compromise on our responsibility to protect the health of our communities, our climate, and the savings consumers stand to gain at the pump,” said the pledge, also signed by California Gov. Gavin Newsom. “We will continue to pursue additional concrete actions to fulfill this duty and defend against any threats.”

Besides California, Puerto Rico and Colorado, leaders of Connecticut, Delaware, Hawaii, Illinois, Maine, Maryland, Massachusetts, Minnesota, Montana, Nevada, New Jersey, New Mexico, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Vermont, Virginia, Washington and Wisconsin signed the mileage pledge.

The Environmental Protection Agency didn’t immediately comment on the pledge, which was made public overnight.

The commitment underscores prospects for years of legal challenges and regulatory uncertainty for automakers if the Trump administration moves ahead on the mileage freeze over objections from California and other states.

President Donald Trump has pushed automakers to support him in his bid to relax mileage standards. Last month, major automakers instead appealed for the administration to return to talks with California, after breaking off negotiations.

Newsom indicated he wasn’t optimistic about any breakthroughs with the administration ahead of it issuing its mileage rule.

“I don’t sense they’re sincere in their commitment to sit down and negotiate,” he said, and cited the administration’s overall backing for the country’s oil and gas industry.

Congress for decades has granted California authority to set its own, tougher mileage standards as a way of fighting the state’s chronic smog. About a dozen states follow California’s mileage standards, although under Obama the federal and state mileage standards were the same.