The Environmental Protection Agency (EPA) last summer proposed reversing the so-called "endangerment finding" of 2009, in the administration's latest boost to the fossil fuel industry.

"On Thursday, President Trump will be joined by (EPA) Administrator Lee Zeldin to formalize the rescission of the 2009 Obama-era endangerment finding," White House Press Secretary Karoline Leavitt told a news briefing Tuesday.

"This will be the largest deregulatory action in American history, and it will save the American people $1.3 trillion in crushing regulation."

The finding under then-president Barack Obama concluded that six greenhouse gases -- including carbon dioxide and methane -- endanger public health and welfare by driving climate change.

That determination flowed from a 2007 Supreme Court decision, Massachusetts v. EPA, which ruled that greenhouse gases qualify as pollutants under the Clean Air Act and directed the EPA to determine whether they pose a danger to public health and welfare.

While it initially applied only to a section of the Clean Air Act governing vehicle emissions, it was later incorporated into other regulations.

As a result, repealing the determination would immediately be accompanied by revoking the requirement for federal greenhouse gas emissions standards for automobiles.

The revocation is also set to place a broader suite of climate regulations at legal risk, including limits on carbon dioxide from power plants and methane from oil and gas operations.

"This action is unlawful, ignores basic science, and denies reality," said Democratic governors Gavin Newsom of California, a likely presidential candidate, and Tony Evers of Wisconsin in a joint statement.

"We know greenhouse gases cause climate change and endanger our communities and our health - and we will not stop fighting to protect the American people from pollution."

- Study authored by climate-skeptics -

The administration's draft proposal, which elicited more than half a million public comments, asserts that greenhouse gases should not be treated as pollutants in the traditional sense because their effects on human health are indirect and global rather than local.

Regulating them within US borders, it contends, cannot meaningfully resolve a worldwide problem.

The proposal also sought to downplay the scale and impacts of human-caused climate change, citing a study commissioned by an Energy Department working group filled with skeptics of human-caused climate change to produce a report challenging the scientific consensus.

That report was widely criticized for misattribution and misstating the conclusions of the studies it cited.

Environmental groups sued the Energy Department, alleging the panel was convened behind closed doors in violation of federal rules.

Energy Secretary Chris Wright later disbanded the group.

- Legal challenges, disputed math -

The Trump administration has claimed that repealing the endangerment finding would lead to reduced new car costs, which have spiraled since the pandemic, but its calculations do not account for savings from reduced fuel use.

Dan Becker of the Center for Biological Diversity's Safe Climate Transport Campaign, said the administration was "stoking oil demand at home by pushing for more gas guzzlers," while presenting a gift to China's electric vehicle makers who would no longer face meaningful US competition.

While the administration says the rules imposed more than $1 trillion in cumulative regulatory costs that were passed on as "hidden taxes," critics say it does not weigh those costs against the monetized benefits from climate protection, public health and fuel savings.

"The EPA's slapdash legal arguments should be laughed out of court," said Meredith Hankins, federal climate legal director at the Natural Resources Defense Council, vowing a swift court challenge.