The Supreme Court’s first big battle over climate change was initiated by states and environmentalists charging that the Environmental Protection Agency under President George W. Bush had abdicated its duty to regulate greenhouse gases.

The sequel is the opposite: Different states and industrialists are charging that the EPA under President Obama has run amok with the power that the court gave it in that first case in 2007.

The new case, which will be heard Monday, has all the makings of a showdown. The court has consolidated a host of challenges; just listing all the parties takes eight pages of one of the briefs. An extra 30 minutes has been scheduled for justices to examine the innards of the Clean Air Act, which has been described as one of the most complicated pieces of legislation on the planet.

The stakes are described in apocalyptic terms. One conservative coalition warns that the Obama administration is attempting “perhaps the most audacious seizure of pure legislative power” since President Harry S. Truman tried to commandeer the steel mills during the Korean War. Environmentalists say the objecting states and business groups are trying to undo decades of EPA practice.

But unless the court decides to revisit its 2007 decision that says the EPA has the power to regulate greenhouse gases — and there’s no evidence the justices are willing to reopen that debate — the upcoming ruling may not live up to the hype.

Utility Air Regulatory Group v. EPA , which deals with “stationary sources” such as power plants and factories, could end up being more about PR than CO2.

Both sides agree that the outcome will not affect the agency’s rules governing emissions from motor vehicles or plans underway to control new power plants. And a victory could be seen as an affirmation of Obama’s authority to move boldly on environmental regulations in the midst of a gridlocked Congress.

But here’s another area of agreement: Even a narrow defeat for the agency would resound.

“Politics are important,” said Richard Lazarus, a Harvard University law professor who is an expert on environmental law. If the EPA loses, “you can be sure the court’s decision will be read as a repudiation of what Obama’s doing.”

Obama and the environmentalists have been winning in court. The most important victory was the 2007 Supreme Court decision in Massachusetts v. EPA, which said that carbon dioxide and other greenhouse gases are pollutants that can be regulated under the Clean Air Act. The case concerned regulating motor vehicles.

The EPA passed those regulations, which were upheld by the U.S. Court of Appeals for the D.C. Circuit in a case the Supreme Court decided not to review.

But the agency said that since greenhouse gases were covered by the Clean Air Act, “stationary sources,” as well as motor vehicles, were subject to the permitting requirements in certain parts of the act. That means factories, existing power plants and other structures.

A unanimous panel of the D.C. Circuit agreed with that view. It said that court precedents made the agency’s readings of its powers “unambiguously correct.” Industry groups asked the entire court to weigh in, and while the judges upheld the panel, a dissent by influential Judge Brett Kavanaugh apparently caught the eye of some Supreme Court justices.

He noted that the agency had revised the emission standard applied to facilities, greatly increasing the amount of greenhouse gases allowed before a source was subject to regulation. Without that change, the EPA said it would have faced “absurd results” with millions of facilities covered. Agencies can interpret laws but they can’t rewrite them, he said.

So the Supreme Court said it accepted the case to decide whether the EPA was right in determining that “its regulation of greenhouse gas emissions from new motor vehicles triggered permitting requirements under the Clean Air Act for stationary sources that emit greenhouse gases.”

Texas Solicitor General Jonathan F. Mitchell, writing on behalf of 12 states, said that the EPA’s actions changing the thresholds show that Congress was not attempting to regulate greenhouse gases.

“The far-reaching and near-ridiculous regulatory burdens required by EPA’s decision to regulate greenhouse-gas emissions . . . prove that the [Clean Air] act never delegated to EPA the authority to regulate greenhouse-gas emissions as ‘air pollutants,’ ” he wrote.

Washington lawyer Peter D. Keisler, representing industry groups, told the court that the EPA has rewritten the law so that it is “targeting only those facilities EPA chooses to regulate and proceeding only on the schedule that EPA decides is warranted.”

But what that means, Solicitor General Donald B. Verrilli points out in the government’s brief, is that fewer facilities are covered and the regulatory burden is lighter.

The EPA is not rewriting the statute, he said, but simply exercising what the Supreme Court noted in the 2007 case was its “broad discretion to choose how best to marshal its limited resources and personnel to carry out its delegated responsibilities,” Verrilli wrote.

Fifteen states, including New York, California and Maryland, are supporting the EPA and saying the permitting process has gone well. And environmentalists and others, such as the liberal Constitutional Accountability Center, say that Congress enacted the Clean Air Act to give the executive branch wide latitude to recognize new threats and the latest science in protecting the environment.

But the CAC’s Tom Donnelly says that a gridlocked Congress has produced what has become a familiar pattern on controversial national issues: “The president acts under the authority granted by an old law, interested parties challenge the president’s actions in court, and for the most important disputes, the Supreme Court gets the final say — with little chance of responsive legislation by Congress.”