Two dozen governors signed a pledge Tuesday to defend Obama-era clean car standards, which President Donald Trump plans to weaken.

The pledge, spearheaded by California Gov. Gavin Newsom, defends former President Barack Obama's standards that would double the fuel economy requirement of new vehicles by 2025.

If Trump gets his way, the nation's vehicles would emit as much as an extra 321 million to 931 million metric tons of carbon dioxide by 2035, The New York Times reported. The Trump administration argues that stricter clean vehicle standards will increase car prices , causing people to keep their older, less-safe vehicles on the road for a longer period of time.

The 24 governors who signed the pledge, including Republicans Charlie Baker of Massachusetts and Phil Scott of Vermont, are ready to fight back.

"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 said in the pledge, according to the Times. "We must unite to ensure a strong, science-based national standard, in California and across the country, that increases year over year.

The governors' statement also defends California's right to create its own clean air standards, which the Trump administration is expected to challenge. Leaders from the following states signed the pledge: California, Colorado, 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 (and Puerto Rico).

The governors aren't the only people concerned about Trump's plans to loosen clean car standards. Automakers have also expressed their opposition, claiming that the auto market would be split since a number of states would follow stricter standards even if the federal rules change.

The governors' statement follows Trump's speech Monday where he said his administration is working "harder than many previous administrations," in fact "maybe almost all of them," to address environmental issues, according to The Washington Post .