The Environmental Protection Agency (EPA) faced legal threats on multiple fronts Friday as environmental groups and utilities filed their first briefs in a high-stakes battle over a Trump administration rollback of power plant regulations that coal companies have also blasted.

Twenty-two states and various environmental groups have challenged the Affordable Clean Energy (ACE) rule, the Trump administration's replacement to the much-litigated Clean Power Plan implemented during the Obama administration.

The ACE rule aims to give states more time and authority to decide how to implement the best new technology to ease net emissions from coal-fired plants. The rule does not set any standards to cap those emissions.

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But environmental groups worry the standards are so limited in the pollution controls it asks power producers to install that if the rule is upheld in court it could hamstring future administrations from addressing climate-altering pollution.

The legal battle kicked off Friday, as the first round of detailed legal briefs were due to the U.S. Court of Appeals, where environmental groups are urging the court to nix the EPA rule.

“The Rule is a blatant abdication of EPA’s statutory duty to protect the public from air pollution that the agency itself has repeatedly found poses grave and imminent dangers to health and welfare,” environmental groups wrote in their brief, part of a coalition between the Environmental Defense Fund, Natural Resources Defense Council (NRDC), Sierra Club and others.

In addition to not capping emissions from power plants, the rule limits regulations to things physically connected to power plant operations.

“This will result in a less than 1 percent reduction in carbon emissions from coal plants and the whole sector compared to what you’d expect with business as usual,” David Doniger with NRDC told The Hill.

“Saying you can’t do anything more than that is just wrong, and we're asking the court to overturn that,” he added.

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Power plants have filed briefs siding with environmental groups, while coal companies have taken another approach, arguing the EPA should not have issued the new regulation at all.

A brief from Westmoreland Mining Holdings and the North American Coal Corporation argues that the EPA doesn’t have the authority to issue the regulation as the law requires a formal finding that coal “causes, or contributes significantly to, air pollution which may reasonably be anticipated to endanger public health or welfare.”

Without such a finding for coal specifically, they argue, the ACE rule should never have been written.

“EPA cannot possibly assess whether sources significantly contribute to ‘air pollution that endangers public health or welfare without identifying the ‘air pollution’ at issue, and certainly EPA cannot establish a meaningful ‘standard of performance’ without identifying the endangering pollutant,” they wrote.