The Environment Protection Agency's climate rules for power plants are the most "audacious" in the history of the agency, argued more than two dozen states, industry, unions, trade groups and others in a massive legal brief filed Friday night in federal appeals court.

The brief is the opening shot in a legal process that will ultimately decide the fate of President Obama's climate legacy.

"EPA's audacious assertion of authority in this rule is more far-reaching than any previous effort by the agency," the mega-brief reads in opposing the agency's Clean Power Plan, the centerpiece of the president's climate agenda.

The plan directs states to cut greenhouse gas emissions a third by 2030, which the plaintiffs say is next to impossible to meet, and will result in the shuttering of much of states' fossil fuel power plants, to be replaced by renewables.

"The reduction requirements can be met only by shutting down hundreds of coal-fired plants, limiting the use of others, and requiring the construction and operation of other types of facilities preferred by EPA — a directive EPA euphemistically calls 'generation shifting,'" the states argue.

The brief was filed by 29 states in the D.C. Circuit Court of Appeals Friday, a week after the Supreme Court stayed the Clean Power Plan until a decision comes down from the appellate court. Friday was the first deadline for plaintiffs in the court's three-month long briefing schedule, before hearing oral arguments in early June. The dozens of plaintiffs suing EPA has grown so vast that the court has slated two days for oral arguments, a rare instance for the D.C. Circuit Court.

The crux of their argument is that "EPA's legal theory is at odds with the plain language of section 111" of the Clean Air Act, "and certainly is not 'clearly' authorized by that provision," the brief reads. Under that section, which is where the Clean Power Plan is based, "EPA may seek to reduce emissions only through measures that can be implemented by individual facilities."

It cannot implement emission reductions through the states directly, but through individual power plants only. "Indeed, for 45 years, EPA has consistently interpreted section 111 standards of performance in this way — not only in the five instances in which it has addressed existing sources, but also in the more than one hundred rulemakings in which it has adopted standards for new sources," the brief adds.

The states also argue that because EPA violates the Clean Air Act, it is overstepping the bounds of its jurisdiction, and violating states' rights and hence the U.S. Constitution.

The next foot to drop in the briefing process will be for EPA to respond to the plaintiffs' arguments, and lay out its legal defense.

EPA is scheduled to file a response on March 28.