The Trump administration’s feud with California has escalated following a threat to withhold federal funding for the state’s highways.

Environmental Protection Agency (EPA) administrator Andrew Wheeler hit out in a letter to California’s Air Resources Board this week slamming the state for having the worst air quality in the United States and failing to address a backlog of air pollution control plans mandated by federal law, the Huffington Post reports.

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“Since the 1970s, California has failed to carry out its most basic tasks under the Clean Air Act. 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 – more than twice as many people as any other state in the country,” the letter reads.

Wheeler said the EPA would commence a “disapproval process” that could impact highway funding if California doesn’t take action on about 130 plans. The Sacramento Bee says California was projected to receive around $19 billion in funding from the Federal Highway Administration between 2016 and 2020.

President Donald Trump has been fighting California for months over the state’s refusal to relax fuel emission standards in line with those announced by the federal government. Last week, Trump revoked California’s ability to set its own car pollution standards. California and 22 other U.S. states have filed a lawsuit against the Trump administration in response alleging that the government is acting in violation of several environmental laws.