LOS ANGELES, July 9 (Reuters) - The state of Ohio on

Tuesday joined oil companies and business groups asking the U.S.

Supreme Court to reverse decisions that underpin California's

ambitious plans to regulate greenhouse gas emissions from cars

and trucks.

The Midwest state joined Valero's Diamond

Alternative Energy and other plaintiffs in challenging

Environmental Protection Agency (EPA) authority under the Clean

Air Act to grant waivers that allow California to set greenhouse

gas emissions limits that are stricter than the federal

government's, after a spate of Supreme Court rulings that weaken

U.S. agency authority.

"The Golden State is not the golden child. Yet in the Clean

Air Act, Congress elevated California above all the other States

by giving to the Golden State alone the power to pass certain

environmental laws," the Ohio plaintiffs wrote in their petition

to the nation's top court.

Ohio's attorney general, who brought the case, and the EPA

did not immediately respond to requests for comment.

In their request for Supreme Court review last week, the

Diamond plaintiffs said it was time for the Supreme Court to

"finally decide whether EPA has the authority to grant

California a preemption waiver to address global climate

change."

The attorney of record for that petition declined comment.

Because it has more cars and some of the nation's worst air

quality, California since the late 1960s has been the only state

with the power to request an EPA waiver to set its own vehicle

pollution standards. For the last two decades, the nation's most

populous state has used waivers to tackle greenhouse gas

emissions from vehicles, which account for about a quarter of

its climate-warming emissions.

EPA granted the California waiver at the center of these two

cases in 2013, during President Barack Obama's administration.

The EPA under President Donald Trump revoked the waiver in 2019

and President Joe Biden reinstated it in 2021.

In addition to targeting California's EPA waivers, opponents

have argued the agency cannot set greenhouse gas emissions

standards from mobile sources like vehicles because it is not

written into the Clean Air Act.

Meanwhile, environmental attorneys are monitoring the

potential impact of two recent Supreme Court decisions.

The 2022 West Virginia v. EPA ruling invoked the "major

questions" doctrine that requires explicit congressional

authorization before regulators can take consequential actions

on issues of vast economic, political and societal impact. In

addition, last week's Corner Post ruling cleared the way for new

lawsuits against rules previously considered settled.

Those cases could set the stage for dismantling rules aimed

at arresting climate change and reducing health impacts on

people living near transportation corridors known as "diesel

death zones," environmental attorneys warned.

"Pretty much everything is up for grabs again," said David

Pettit, senior attorney for climate and energy with the Natural

Resources Defense Council.

Ultimately, California waiver challenges may turn on whether

opponents can convince the court that the state is being

arbitrary and capricious or that it does not need such standards

"to meet compelling and extraordinary conditions," said Stan

Meiburg, a former acting EPA deputy administrator under Obama.