Nearly half of U.S. states have joined a lawsuit seeking to block President Donald Trump’s administration from lowering fuel economy standards set by his predecessor, Barack Obama.

In a suit filed with the Washington D.C. U.S. Court of Appeals, states asked the court to review Trump’s plan to reduce annual fuel economy targets to about 1.5% increases from 5% jumps. And, the states are challenging the administration’s technical review used as a justification for the new rules.

“For 50 years, California has led the way by requiring automobile innovations that reduce the health impacts of tailpipe pollution,” says California Environmental Protection Agency Secretary Jared Blumenfeld. “The Trump Administration wants to take us back to a time of increased air pollution, which will have devastating effects on public health. We can’t afford any more kids with asthma – especially given the impacts of the current COVID-19 pandemic. California will continue fighting for a clean air future.”

Trump’s rollback plans were destined for the courtroom since he announced his intention to revisit the Obama standards soon after winning the 2016 election. Part of the president’s plan is to cancel a waiver that California receives from the U.S. Environmental Protection Agency (EPA) that has allowed it to set higher standards than federal fuel economy rules. States representing nearly half of U.S. drivers follow those California rules, effectively creating two regulatory systems in the country.

The Obama-era policies were set to unify standards by setting a much higher federal target – 54.5mpg by 2025 (although with technical credits, that number falls to about 43mpg). However, automakers have argued that shifting buying habits in the past several years make these higher goals unobtainable – since consumers are opting for more trucks, crossovers, and SUVs and fewer fuel-sipping passenger cars.

Ford, Honda, Volkswagen, and BMW pledged to meet the higher California standards, even if Trump’s efforts prevail – promises that the president called illegal collusion and urged the Department of Justice to investigate. General Motors and other automakers pledged support for higher standards but argued against keeping the Obama-era rules.

Last year, the administration proposed cancelling California’s waiver and freezing standards at about 37mpg (after credits for air conditioning improvements and other systems). Officials later revised that to a 1.5% annual increase in efficiency.

“The federal agencies used questionable science, faulty logic, and ludicrous assumptions to justify what they wanted from the start: to gut and rewrite the single most important air regulation of the past decade,” says California Air Resources Board Chair Mary D. Nichols. “Despite all the hype, this rule is a loss for America, as evidenced by their own analysis.”

Several cities also joined the suit including Los Angeles, New York, and the cities and counties of San Francisco and Denver.

California Air Resources Board