The Trump administration, taking aim at one of former President Barack Obama’s signature environmental achievements, is proposing to freeze vehicle fuel economy requirements at the 2020 level and unwind California’s authority to limit tailpipe emissions in the state.

The Environmental Protection Agency and National Highway Traffic Safety Administration jointly proposed on Thursday to cap fuel economy requirements at a fleet average of 37 miles per gallon starting in 2020. Under the Obama plan, the fleetwide fuel economy would have risen gradually to roughly 47 mpg by 2025.

They also propose to revoke California’s authority under the Clean Air Act that has allowed the state to set rules more stringent than the federal ones limiting tailpipe greenhouse gas emissions as well as an electric-vehicle sales mandate.

Taken together, the proposal is an aggressive move to dismantle what some environmentalists have hailed as one of the most potent efforts anywhere in the world to combat climate change. Yet for President Donald Trump, who’s prioritized eliminating regulations, the auto rules represent a grand prize.

Release of the proposal begins a 60-day period of public comment that is sure to attract a spirited response from California’s leaders who have vowed to defend their smog-fighting authority in court and from environmentalists who view the plan as a setback to efforts to curb climate change. Automakers, who had asked to be relieved of some of the mandates, have expressed misgivings about having to accommodate a patchwork of federal and state standards.

“There are compelling reasons for a new rulemaking on fuel economy standards for 2021-2026,” Transportation Secretary Elaine Chao said in a statement. “More realistic standards will promote a healthy economy by bringing newer, safer, cleaner and more fuel-efficient vehicles to U.S. roads and we look forward to receiving input from the public.”

Trump in March 2017 announced his administration would be reopening a review of the Obama automobile rules that he and some automakers said was unfairly cut short by Obama’s EPA. Before a crowd of autoworkers assembled at a decommissioned car factory, Trump said “we’re going to work on the CAFE standards so you can make cars in America again. We’re going to help the companies, and they’re going to help you.”

Thursday’s rule-making proposal from the EPA and NHTSA also presents several other options for modifying the Obama targets, while recommending the proposed freeze starting in 2020, the most severe of the scenarios.

In a separate move to unwind California’s power to set the pace of auto emissions requirements, the proposal also asserts that a 1975 law prohibits states from setting their own greenhouse gas limits, a view two federal district judges have already rejected.

A 60-day comment period will begin once the proposal is published in the Federal Register, and a final rule could be finalized by the end of 2018 or early next year.

In Thursday’s proposal, the Trump administration argues that its proposed freeze will have a “negligible’’ impact on air quality, and boost the earth’s temperature by 3/1000th of one degree Celcius by 2100. The administration also says that its preferred plan for fuel economy will reduce society-wide spending by $502 billion for vehicles built between 1975 and 2029.

About a third of these projected savings, or $198 billion, are tied to the agencies’ assertion that, by slowing pace at which new vehicles get more expensive, they’ll allow people to replace older and less-safe cars more rapidly. The agencies say this accelerated fleet turnover will reduce the cost of accidents and avoid 12,700 traffic deaths in vehicles built through 2029. Not everyone’s convinced.

"This would be utterly unprecedented. Nobody in any regulatory agency has ever done a cost-benefit analysis anything like this,’’ said Nic Lutsey, an engineer at the International Council on Clean Transportation. “There’s no evidence that efficiency regulations have depressed sales and added fatalities as a result--in any market in the world.’’

The efficacy of the administration’s cost estimates will shape the plan’s public acceptance and its ability to withstand court challenges. California, 16 states and the District of Columbia have already filed a lawsuit that challenges the plan’s scientific underpinnings.