Updated 3:34 p.m. ET

The Trump administration has escalated its fight with California over environmental regulations.

Environmental Protection Agency Administrator Andrew Wheeler sent a letter Monday to the California Air Resources Board threatening to withdraw billions of dollars in federal highway money unless the state clears a backlog of air pollution control plans.

"California has the worst air quality in the United States, with 82 nonattainment areas and 34 million people living in areas that do not meet National Ambient Air Quality Standards," Wheeler wrote.

California's population, topography, weather and other factors do worsen air pollution there, but the state also is widely recognized as a leader in improving air quality.

Under the Clean Air Act, states that don't meet federal air quality standards are required to submit "State Implementation Plans." Wheeler says California represents a disproportionate share of the backlog of such plans.

"Since the 1970s, California has failed to carry out its most basic tasks under the Clean Air Act," said Wheeler. He then laid out the penalties for failing to meet federal requirements, which include losing federal highway dollars. California is expected to receive about $8 billion in fiscal years 2019 and 2020.

The Trump administration has been picking fights with California over environmental regulations recently. Last week the administration said it will revoke a waiver that allows California to set stricter car emission standards.

A senior EPA official said the two actions were not linked and that California is the focus now because it represents the largest share of backlogged plans. Even though other states have similar backlogs, the administration has not sent letters to them.

Gov. Gavin Newsom has been critical of President Trump's environmental record and challenged the administration in court. Newsom called Wheeler's letter a "brazen political stunt."

"The White House has no interest in helping California comply with the Clean Air Act to improve the health and well-being of Californians. This letter is a threat of pure retaliation," Newsom says.

Former EPA officials say they were surprised Wheeler sent the letter to California.

"I just think it's so ironic that the EPA is chastising California, which has been so progressive in working towards cleaner air, more than any other place in the country," says Janet McCabe, who was an EPA official during the Obama administration and now directs Indiana University's Environmental Resilience Institute.

McCabe says it's unusual for the EPA to publicly criticize a state in this way. She says typically the agency would communicate with a state privately and work out differences to encourage a cooperative rather than adversarial relationship.

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