The Trump administration has drafted a proposal that would freeze fuel-efficiency standards for automobiles starting in 2021 and challenge California’s ability to set efficiency rules of its own, a change that would hobble one of the Obama administration’s most significant initiatives to curb climate change.

The draft document, while not final, suggests the Trump administration is poised to make significant changes to planned auto standards over the next decade. A federal official, who has reviewed the document, described it in detail to The Washington Post.

Drafted in large part by the Department of Transportation’s National Highway Traffic and Safety Administration (NHTSA), the plan outlines a preferred alternative where the federal government would freeze fuel efficiency standards for cars and light trucks at levels now set for Model Year 2021, keeping them there through 2026.

The draft offers seven other options that would also weaken the standards, though not to the same extent as the preferred alternative.

Under an agreement reached between the Obama administration, California officials and automakers several years ago, manufacturers’ fleet of cars and light trucks in the U.S. are slated under current rules to average more than 50 miles per gallon by 2025 – well above the levels at which the Trump administration is proposing to freeze the standards.

If finalized, the Trump administration’s proposal would set the stage for a major conflict with California, one of the nation’s most progressive states on climate change and air pollution.

The Obama administration granted California a waiver under the Clean Air Act to set its own tailpipe emissions limits, and the state’s higher standards have led automakers to build more fuel-efficient automobiles to maintain access to California’s massive market. But the Trump administration document asserts that, despite the Clean Air Act waiver, a separate federal law preempts California from drafting its own emissions standards.

Earlier this month, Environmental Protection Agency Administrator Scott Pruitt announced he would revoke the Obama-era standards, but he did not specify what would take their place. Pruitt concluded they were “not appropriate” in light of new information, including automakers’ input that consumer demand for sport-utility vehicles and pickup trucks far outweighed interest in electric and other low-emissions vehicles.

Pruitt has publicly hinted dissatisfaction with California’s more stringent auto standards, though in other instances he has argued that states should have more discretion in crafting environmental rules.

“Federalism doesn’t mean that one state can dictate to the rest of the country,” Pruitt told members of the Senate Environment and Public Works Committee in January.

When asked again on Thursday if the EPA intends to start proceedings to revoke California’s waiver, Pruitt told the House Energy and Commerce subcommittee on the environment: “Not at present. In fact, we’ve worked very closely with California officials on that issue.”

Stanley Young, a spokesman for the California Air Resources Board, took exception to Pruitt’s characterization of relations between the two government agencies. “Pruitt himself has never met with anyone from CARB – even when he was in California in March,” Young wrote by email. He added that EPA and CARB officials have had three “nonsubstantive” meetings over the past four months. “This is not, by any stretch of the definition, ‘working with California,’ ” Young said.

California’s top prosecutor hinted Friday it may challenge the EPA’s new auto standards in court, should they come to fruition.

“The Trump Administration’s plan would rob Americans at the gas pump and risk our childrens’ health by polluting the air we breathe,” California Attorney General Xavier Becerra said in a statement. “We’ll closely monitor any developments and I’m ready to take any and all action necessary to defend our progress.”

The current standards were set to avoid 6 billion tons of carbon dioxide emissions from vehicles sold between 2012 and 2025, according to the EPA. Since the rules were issued, the transportation sector has outstripped electric power to become the top source of greenhouse gas emissions in the United States.

Some automakers such as Ford and Honda have publicly cautioned against a rollback of the current national tailpipe limits. Any proposal to reverse the existing limits would be subject to public comment before being finalized, and a protracted court battle could delay the change even further.

National Highway Traffic and Safety Administration officials Friday stressed in an emailed statement to the Post that the plan was not final, promising the upcoming review process would be “public, robust, and transparent.”

“NHTSA and EPA continue to work together on Corporate Average Fuel Economy and tailpipe standards for future model year passenger cars and light trucks. NHTSA’s top priority is safety and this Administration must also consider economic practicability when setting these Standards,” they wrote. “The agencies intend to take comment on a broad range of options. Given that the work is ongoing, at this time there is nothing to announce until a proposal is actually released.”

The Environmental Protection Agency has not signed off on the draft, and the plan has not yet been sent to the White House for review.

“The Agency is continuing to work with NHTSA to develop a joint proposed rule and is looking forward to the interagency process,” EPA spokesman Liz Bowman said in an email.

Democrats immediately pushed back on the Trump administration proposal.

“Rather than pursuing a reasonable compromise, the Trump Administration is crafting a proposal that is dramatically weaker than any automobile manufacturer has requested and that also deliberately seeks to embark on a legal collision course with the State of California – a scenario that automakers, lawmakers and the State of California have all repeatedly urged the administration to avoid,” Senator Tom Carper, D-Del., said of the new document in a statement.