WASHINGTON—As the Trump administration moves to roll back ambitious vehicle-emissions targets, California and several other states that rely on those standards to achieve their clean air goals have enlisted an influential new partner in the fight to keep the rules intact.

Colorado Gov. John Hickenlooper announced Tuesday that his state plans to join a dozen others that will refuse to go along with any rollback. Instead, it will join the other states in invoking a provision under the Clean Air Act that allows them to follow California in continuing to pursue the aggressive goals set during the Obama administration. Those goals aim to have cars and SUVs rolling onto showroom floors averaging 55 miles per gallon by 2025.

Colorado Gov. John Hickenlooper speaks at the state Capitol in May. On Tuesday, Hickenlooper ordered state regulators to adopt California's vehicle pollution rules, joining other states in resisting the Trump administration's plans to ease up on emissions standards. Associated Press/David Zalubowski

Colorado’s move intensifies the fight between the states and the administration, which is positioning to strip California of its authority to set emissions rules that are tougher than those imposed by the Environmental Protection Agency.

The administration has not yet publicly released its plan, but a draft reviewed by several federal officials who shared details with the Los Angeles Times calls for freezing the mileage standard in place for six years. Under that plan, the coalition of states led by California – which accounts for more than a third of all vehicles sold nationwide – would be pre-empted from imposing their own rules. The administration’s blueprint could make it impossible for several states to meet their climate goals, as auto emissions are now the No. 1 source of greenhouse gases nationwide.

The federal vehicle mileage targets are considered the nation’s single most effective program in the fight against climate change.

California and the other states say any attempt by the administration to block them from keeping the Obama-era targets would be illegal. Colorado promises to be a potent ally in that fight. The state is politically influential, and it also now will become the first state in the nation’s interior to embrace the tailpipe emissions crusade.

Colorado officials say they are aligning with California because, like that state, Colorado is uniquely vulnerable to air pollution because of its topography. The state has also set ambitious goals for greenhouse gas reduction, aiming to lower the emissions by 26 percent by 2025.

“Low emissions vehicles are increasingly popular with consumers and are better for our air,” Hickenlooper said in a statement after signing an executive order that puts the state on a path toward adopting the California standards by the end of the year. “Every move we make to safeguard our environment is a move in the right direction.”

Environmental groups were uplifted by the governor’s move.

“With the Trump administration abdicating leadership on cleaning up tailpipe pollution and saving consumers money on gas, states need advanced vehicle standards to ensure their citizens get to drive the cleanest, most affordable cars on the market,” said Noah Long, senior attorney for the Natural Resources Defense Council. “This action will help ensure Coloradans still get clean air and cleaner cars.”

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