Toyota usually has a soft spot for California. The automaker has promoted its Prius hybrid in ads as “redesigned, CA style,” and shown its sedans weaving through verdant Golden State scenery as a Beach Boys song plays.

But the good vibrations between Californians and Toyota may be receding this week after the automaker sided against the state in an increasingly heated fight over fuel economy standards for automobiles. Toyota backed the Trump administration by saying that the federal government’s authority should supersede the state’s in setting emissions limits.

Toyota’s support shocked consumers in a state known for its environmental activism, especially those who had seen the company as a leader in clean-car manufacturing. Some said they would no longer buy Toyota vehicles, opting instead for competitors like Ford, Volkswagen, Honda or BMW — all of which reached a surprise agreement with California in July to scale back emissions in their cars.

“What are they thinking?” said Jeff Goodby, a co-founder of the Goodby, Silverstein & Partners ad agency in San Francisco, which handles campaigns for BMW. “I can’t imagine any Californian saying, ‘All things being equal, I’m buying the brand that spews more poison into our children’s air!’”

The state has vowed to abide by an Obama-era standard that would require automakers to build vehicles that reach an average of 54.5 miles per gallon by 2025, eliminating about six billion tons of carbon dioxide from the atmosphere over the vehicles’ lifetime. The Trump administration wants to loosen the regulation to about 37 miles per gallon.

Costanza Rampini, an assistant professor of environmental studies at San Jose State University, wrote on Twitter that she was “furious” with Toyota for aligning itself with President Trump. She has owned a black Prius for more than a decade but wrote that she plans to swap it out for a Ford or Honda vehicle as soon as possible.

“You are on the wrong side of history Toyota,” she wrote, adding in an interview that the move was “really a punch in the gut for all of the Toyota customers that have been supporting the company as a champion at the avant-garde of low-emission cars.”

Marketing experts said that Toyota would most likely be penalized by consumers more than General Motors and Fiat Chrysler, which also sided with the administration on Monday. While both G.M. and Fiat Chrysler have made large investments to develop hybrid and electric cars, their environmental bona fides aren’t as prominent as Toyota’s.

“The brand perception of Toyota is quite at odds with this view,” said Puneet Manchanda, a marketing professor at the University of Michigan. “If any brand is going to be hurt, Toyota would probably be the most.”

Toyota did not respond to a request for comment. Publicis, which owns the ad agencies responsible for much of Toyota and G.M.’s marketing, declined to comment.

As the emissions debate divides automakers in California, those that have struggled to develop their environmental credentials may now find an opening to play up their commitments to limiting pollution. Volkswagen, after addressing its emissions cheating scandal in print and video ads this summer, has since turned its focus to marketing its coming lineup of electric vehicles.

The automotive industry spends heavily on advertising, devoting an estimated $35.5 billion across 14 global markets last year, according to research from the Zenith media agency. This year, automakers will be second only to retailers in digital ad spending, according to eMarketer, the research firm.

But so far, they’ve barely touched on environmental issues.

Five automakers advertised during the Super Bowl this year; only Audi and Toyota explicitly addressed electric or hybrid vehicles.

Advertising for electric vehicles is more common in Europe, where environmental regulations are more established.

Toyota is most likely betting that consumers’ memory is short, and that pushback will be limited, experts said. And environmental concerns don’t seem to be the main reasons most people buy cars.

Only 7 percent of the 8,000 people surveyed recently by Brandwatch, a data firm, said that sustainability was an important issue when choosing auto brands. A Deloitte survey found that while 29 percent of respondents said they would prefer a hybrid, battery-powered or other alternative vehicle, up from 20 percent last year, demand may be dampened by low fuel prices, relaxed emissions standards and fewer rebates.

“Safety is a much bigger concern than ideology,” Mr. Manchanda said.