By David Shepardson

WASHINGTON (Reuters) – The Trump administration escalated its fight with California on Tuesday, accusing the state of failing to enforce the U.S. Clean Air Act and threatening to withdraw billions of dollars in federal highway funds to the country’s most populous state.

California, which has imposed strict state standards limiting vehicle emissions in defiance of Trump’s attempts to roll back regulations, has “the worst air quality in the United States,” U.S. Environmental Protection Agency (EPA) Administrator Andrew Wheeler wrote to California Air Resource Board chief Mary Nichols in a letter dated Tuesday.

EPA officials acknowledged they had not sent any similar letters to other states and denied the letter was politically motivated. They say California has the largest backlog of state implementation plans to address ambient air quality standards.

The letter contended that California “has failed to carry out its most basic tasks under the Clean Air Act” and 34 million people in the state live in areas that do not meet air quality standards “more than twice as many people as any other state.”

Wheeler said in a statement “EPA stands ready to work with California to meet the Trump Administration’s goal of clean, healthy air for all Americans, and we hope the state will work with us in good faith.”

California is set to receive just over $4 billion in federal highway funding in the budget year that starts Oct. 1.

Senator Dianne Feinstein, a California Democrat, said the “threat to withhold California’s highway funding over clean air quality reports is the height of hypocrisy. California doesn’t need to be lectured by an administration beholden to polluters.”

She added that “the administration is trying to undo all the progress we’ve made in reducing harmful emissions and combating climate change.”

Last week, the EPA said it was taking the unprecedented step of revoking California’s waiver under the Clean Air Act to set tailpipe emissions standards and require zero emission vehicles, while the National Highway Traffic Safety Administration (NHTSA) said California was preempted from take action on tailpipe emissions.

On Friday, California and 22 other states sued NHTSA and it plans to challenge the EPA decision at a later date. The letter was reported on Monday by the Sacramento Bee but was to be announced by the EPA on Tuesday.

Earlier this year, the Trump administration said it was withholding $929 million from California’s high-speed rail project that was awarded in 2010, prompting the state to file suit.

On Tuesday, the state of California filed its 29th environmental lawsuit against the Trump administration, challenging the EPA’s determination that 1,365 acres (552 hectares) of salt ponds in Redwood City are not “waters of the United States” under the Clean Water Act.

(Reporting by David Shepardson; Editing by David Gregorio and Marguerita Choy)