FRESNO, Calif. (AP) — Electric vehicle advocates, environmental groups and California officials blasted the Trump administration's proposal to roll back car-mileage standards at a Monday hearing in a region with some of the nation's worst air pollution.

The session in California's Central Valley is the first of three events by the U.S. Environmental Protection Agency and National Highway Traffic Safety Administration to gather public comment on the mileage plan.

The proposal announced in August would freeze U.S. mileage standards at levels mandated by the Obama administration for 2020 instead of continuing to rise to 36 miles per gallon (15 kilometers per liter) by 2025, 10 miles per gallon (4 kilometers per liter) higher than the current requirement.

Administration officials say waiving the tougher fuel efficiency requirements would make vehicles more affordable, which would get safer cars into consumers' hands more quickly.

Opponents say it would undercut efforts to reduce unhealthy tailpipe emissions that are a significant contributor to climate change. California was a key player in arriving at the Obama-era mileage standards that would be reversed.

California Attorney General Xavier Becerra and California Air Resources Board chairwoman Mary Nichols were among several state officials on the first panel to testify.

Nichols said the administration's claims that the rollback would improve safety were absurd.

"We will not sit idly by as you propose to flatline our efforts," she said. "We must continue to insist on cars that produce fewer emissions, including millions more zero-emission vehicles."

Becerra said California could not afford to retreat in the fight against climate change, noting the wildfires that have ravaged the state over the last several years.

"My message to the federal government: 'Do your job. Withdraw this proposal,'" Becerra said. The audience applauded Becerra and Nichols' comments.

Environmentalists protested outside the hall where the hearing took place, hoisting signs that read "Clean cars = Clean air" and "Clean cars now."

Paul Gipe, 67, and his wife Nancy Nies, 69, drove from the city of Bakersfield to join the demonstration against the rollback.

"It's a step backward and it's a statement that air pollution is acceptable. Damn the people, full speed ahead," said Gipe, who writes about renewable energy on his website.

An avid bicyclist, Gipe said there are days he can't ride because the air quality is so bad in his hometown.

California and other states have filed a lawsuit to block any changes to the fuel efficiency rules. The administration also wants to revoke California's authority to set its own mileage standards.

Most automakers won't state explicitly that they favor freezing the requirements at the 2021 level, which is the Trump administration's preferred option.

But in testimony prepared for the Fresno hearing, Steve Douglas, senior director of energy and environment for the Alliance of Automobile Manufacturers, which represents General Motors, Ford, Fiat Chrysler, Volkswagen, BMW and other automakers, said customers aren't buying more efficient vehicles.

"No one wins if our customers are not buying the new highly efficient products offered in our showrooms," the written testimony said. "The standards must account for consumer willingness and ability to pay for newer technologies in order for all the benefits of new vehicles to be realized."

The alliance has said there's a widening gap between the requirements approved by the Obama administration and what consumers are buying.

But Ford CEO Jim Hackett said in a speech last week that his company is against any freeze and favors "keeping the standard, not a rollback, we have plans to meet it."

Automakers are unanimous in favoring one standard for the whole country so they don't have to design two vehicles, one for California and the states that follow its requirements and another for the rest of the nation.

The hearing is taking place in the San Joaquin Valley, the country's most productive agricultural region but an area that is plagued by sooty air in part because of its bowl-shaped geography. The Sierra Nevada and two other mountain ranges wall in the 250-mile (400-kilometer) valley.

The air pollution there is blamed for hundreds of deaths each year.

The Obama administration had planned to keep toughening fuel requirements through 2026, saying the stricter standards would save lives.

Trump administration officials argued they would raise the price of vehicles by an average of more than $2,000, preventing many people from buying newer, safer and cleaner vehicles. Transportation experts have challenged those arguments.

Hearings are also planned on Tuesday in Dearborn, Michigan, and Wednesday in Pittsburgh.

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Auto Writer Tom Krisher contributed this report from Detroit.