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. (Reporting by Lisa Baertlein in Los Angeles and Valerie Volcovici and David Shepardson in Washington; Editing by Sandra Maler and Jamie Freed)