Congress took another step to erode California’s ability to regulate emissions affecting air quality, dealing a potentially significantly blow to Tesla’s ability to profit from sales of regulatory credits for producingzero-emission vehicles.

Traffic moved on the San Francisco-Oakland Bay Bridge on Monday.

California has long had a waiver from the Environmental Protection Agency that allowed it to regulate its own air quality. But on Thursday morning, the Senate voted to essentially eliminate that, using the Congressional Review Act, a law that allows legislators to overturn the actions of federal agencies.

The California Air Resources Board, which sets emission policy for the state, didn’t immediately respond to a request for comment.

Democratic lawmakers warned that set a bad precedent for upending agency decisions. “The Republican plan would backfire,” said Ron Wyden (D., Ore), the ranking member of the Senate Finance Committee, in a news release. “Partisan actions cut both ways.”

That means Democrats could do the same thing to federal rules favored by Republicans down the road.

The vote is just one twist in a continuing fight over California emissions. California has successfully fought to keep its ability to regulate air emissions in the past, and it has support in the current battle.

“Today, in an unprecedented and illegal attack on the nation’s bedrock air quality law, the United States Senate passed Congressional Review Act (CRA) resolutions to block states from implementing policies to reduce cancer-causing pollution from motor vehicles,” said CALSTART, a nonprofit advancing clean transportation technologies, in an emailed statement. “Authorization for these policies has been a core component of the Clean Air Act for more than 50 years.”

Earlier in May, House lawmakers voted to nullify a so-called waiver of preemption permitting California’s “Advanced Clean Cars II” regulations that the EPA had posted in January.

Advanced Clean Cars II currently requires roughly two-thirds of all vehicles sold in California—and several other states that follow California’s standards—to be zero-emission by 2030. That is a tall order because EVs account for about 20% of California’s new car sales today. They are only about 7% to 8% nationwide.

The Alliance for Automotive Innovation, which includes General Motors and Ford Motor, supports rolling back the 2030 requirement.

The California regulations form the basis for much of Tesla’s sales of zero-emission vehicle credits, which it receives because its all-electric fleet allows it to far exceed targets set by regulators. Sales of those credits to auto makers that haven’t met the targets have generated about $2.9 billion over the past 12 months.

Tesla stock, so far, has been relatively immune to the California news. Tesla stock closed up 1.9% on Thursday, despite the vote, while the S&P 500 and Dow Jones Industrial Average were both edged lower.

One explanation for the move is that investors are increasingly viewing Tesla as an artificial-intelligence company than a car manufacturer. Investors are focusing on the company’s plans to launch a robo-taxi service in Austin, Texas, in June—an enterprise that depends more on Tesla’s AI-trained self-driving software than its electric motors and batteries.

Tesla investors might also realize there is still a long way to go to fully overturning the California system.