WASHINGTON — The Trump administration is expected Tuesday to announce its final rule to roll back Obama-era automobile fuel efficiency standards, relaxing efforts to limit climate-warming tailpipe pollution and virtually undoing the government’s biggest effort to combat climate change.

The new rule, written by the Environmental Protection Agency and the Department of Transportation, would allow vehicles on U.S. roads to emit nearly 1 billion tons more carbon dioxide over the lifetime of the cars than they would have under the Obama standards and hundreds of millions of tons more than will be emitted under standards being implemented in Europe and Asia.

Trump administration officials raced to complete the auto rule by this spring, even as the White House was consumed with responding to the coronavirus crisis. President Donald Trump is expected to extol the rule, which will stand as one of the most consequential regulatory rollbacks of his administration, as a needed salve for an economy crippled by the pandemic.

The lower fuel-efficiency standard “is the single most important thing that the administration can do to fulfill President Trump’s campaign promise of reforming the regulatory state, and to undo the impact that the previous administration has had on the economy,” said Thomas J. Pyle, president of the Institute for Energy Research, an organization that supports the use of fossil fuels.

Trump’s critics said the rule showed the president’s disregard for science and could actually harm the economy over time. The administration’s own draft economic analyses of the rule showed that it could hurt consumers by forcing them to buy more gasoline. And a February report by a panel of government-appointed scientists, many of them selected by the Trump administration, concluded that “there are significant weaknesses in the scientific analysis” of the rule.

“This is not just an inopportune moment to finalize a major rule-making,” said Sen. Thomas R. Carper of Delaware, the ranking Democrat on the Senate Environment Committee. “In this case, it’s a completely irresponsible one.”

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Even many large automakers, which had asked Trump to slightly loosen the Obama-era rule, had urged him not to roll it back so aggressively because the measure is certain to get bogged down in court for years, leaving their industry in regulatory limbo.

“The auto industry has consistently called for year-over-year increases in fuel efficiency,” said John Bozzella, president of the Alliance for Automotive Innovation, a lobbying group that represents the world’s largest auto companies. “We need a policy environment that drives improvements in fuel economy, and the infrastructure that supports a transformation to net-zero emissions.”

The new rule, which is expected to be implemented by late spring, will roll back a 2012 rule that required automakers’ fleets to average about 54 mpg by 2025. Instead, the fleets would have to average about 40 mpg. To meet the new number, fuel economy standards would have to rise by about 1.5% a year, compared with the 5% annual increase required by the Obama rule. The industry has said it would increase fuel economy standards by about 2.4% a year without any regulation.

The new standard would lead to nearly 1 billion more tons of planet-warming carbon dioxide released and the consumption of about 80 billion more gallons of gasoline over the lifetime of the vehicles built during the terms of the rule, according to a recent draft of the plan.

For Trump, completing the rollback of his predecessor’s climate change policy caps a three-year effort to weaken or undo nearly 100 rules and regulations that had limited industrial pollution of smog, toxic chemicals, greenhouse gases and water contaminants.

Businesses said those rules had cut their growth and left them less competitive in a global economy. Environmentalists said they were vital to the health and future of the planet and its inhabitants.

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For environmentalists, the tailpipe-emissions rule culminates Trump’s quest to dismantle the United States’ efforts to fight climate change, which has so far included pulling out of the global Paris climate change accord, weakening rules to curb greenhouse gas emissions from coal-fired electric plants and undermining the basic science that underpins environmental regulations.

The regulation on vehicle fuel economy, first issued in 2009 and updated in 2012, was the first federal policy to lower emissions of greenhouse gases that cause climate change, and it remains the policy that would have reduced emissions the most, chiefly by targeting the nation’s largest source of climate-warming emissions: cars and trucks.

“In the march of rollbacks of climate rules, this is the biggest step,” said David Victor, director of the Laboratory on International Law and Regulation at the University of California, San Diego.

The new rule creates short-term regulatory relief for automakers, lifting requirements that had forced them to invest heavily in developing and marketing hybrid, electric and low-pollution vehicles.

But in the long run, analysts say, the new standard could actually bring more uncertainty to the auto industry. The legal status of the new Trump rule remains unclear; about 20 states are expected to sue the administration to undo it in a case expected to be resolved by the Supreme Court in the coming years.

Trump administration officials have been plagued with difficulties and delays as they have tried to write a rule that would meet the president’s desire for a drastic regulatory rollback that also would be scientifically sound and legally defensible.

The chief cause of the delay has been an internal economic analysis concluding that the rule would harm consumers more than helping them. A draft of the rule sent to the White House in January calculated that the new fuel economy target would lower the prices of new cars and light trucks by about $1,000, but it would increase the amount consumers would pay for gasoline by about $1,400.

The administration’s draft analysis concluded that the rule could actually cost the U.S. economy between $13 billion and $22 billion.

As administration officials sought to rework those numbers to show that the rule would help consumers, automakers pushed the White House to complete the rule by March 30, the deadline needed to begin manufacturing vehicles under the new standard for the 2022 model year.

Over the weekend, White House officials looked at a new option for their cost-benefit analysis, according to two people familiar with the matter, who asked to speak anonymously because the rule was not yet final. Their solution was an approach that factors in the estimated costs of building fuel-efficient vehicles that are, so far, less popular with consumers. By eliminating the Obama-era mandate to build and sell such vehicles, the thinking goes, automakers would see an economic benefit in the range of $38 billion to $58 billion.

Some analysts scoffed at that assertion.

“It’s not going to be supported by the science,” said Chet France, a former senior EPA vehicle emissions specialist who is now a consultant who works with the Environmental Defense Fund, an advocacy group. “This will be the icing on cake of the legal flaws.”

If the rule does survive legal challenges, it would put the United States out of step with the rest of the global auto market, moving it from one of the strongest fuel economy standards in the world to one of the weakest. The U.S. standard would be lower than those of the European Union, China, India, Japan and South Korea. That could present long-term challenges to the U.S. auto industry, as other automakers develop more sophisticated, high-efficiency, low-pollution vehicles while American ones focus on gas guzzlers.

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The new measure will combine with a related rule, issued last year, that revoked the right of California and other states to set their own tougher vehicle emissions standards. California has spearheaded a pair of multistate lawsuits to block both rules, escalating a political war between Trump and the nation’s most populous state.

Ultimately, the limits on atmospheric emissions will likely be decided by the Supreme Court. Automakers most fear verdicts that uphold the legal authority of California and other states to enact their own standards and allow the Trump standards to move forward elsewhere. That would split the nation’s auto market in two.

“One thing we’ve learned from the Trump administration is be careful what you ask for,” said Victor of the University of California, San Diego.

“The auto industry wanted a smoother glide path to a more efficient future,” he added. “Instead what they got was the populist politics of the far-right, which is blowing up in their faces.”

Trump’s defenders say his moves are not aimed at automakers but at consumers who generally prefer larger vehicles, such as pickup trucks and SUVs.

“He’s not doing it to be conciliatory to industry, he’s doing it for consumers and for his own personal and political reason,” Pyle said.