For years, Oregon has been working to write and enforce rules to reduce greenhouse gas emissions from commercial trucks. Now the Trump administration and the U.S. Senate could derail that effort.

Oregon’s clean trucks rules sought to reduce greenhouse gas emissions from medium- and heavy-duty trucks and to increase sales of electric trucks over the next ten years. But last week Congress voted to revoke a waiver that allowed California to adopt stricter vehicle standards – and that allowed Oregon and other states to follow that lead.

Oregon environmental advocates say losing this waiver could set back the progress the state has made to reduce carbon emissions from the transportation sector. They’ve called on the state to implement its own rules – although it’s not clear if that’s allowed under federal law.

Under the U.S. Clean Air Act, California can set air pollution standards that are stricter than the federal government’s. States, including Oregon and others, can then follow suit, choosing to identically adopt some or all of California’s stricter standards through the Clean Air Act’s Section 177.

Now, those waivers are in limbo, after the U.S. Senate voted to overturn California’s electric vehicle and clean trucks rules.

California’s Gov. Gavin Newsom announced the state will file a lawsuit to challenge that vote.

The rules have not been universally popular in Oregon, which is home to Daimler Truck North America. In January, Portland-based Daimler briefly halted diesel-powered truck sales in the state over a misunderstanding with regulators. After meeting with DEQ to clarify the misunderstanding, the company resumed Oregon sales.

But trucking groups, including advocates and associations in Oregon, have called the standards set by California and adopted by the state “unachievable.”

Environmental groups have largely supported the rules.

Mary Peveto, co-director of Oregon advocacy group Neighbors For Clean Air, called losing the waiver that allows Oregon to follow California’s environmental lead a devastating blow. It’s been “the vehicle of progress and innovation in our country” and integral for the state to reduce emissions from the transportation sector, which accounts for 35% of the state’s carbon emissions, Peveto said.

“The question is, can our state hold the line? Can our state Legislature, with a supermajority of Democrats, decide that it can take a stand against this national attempt to undermine the very principles of our most effective environmental policies?”

Peveto said every advancement Oregon has made to reduce air pollution from the transportation sector has been driven by the state adopting California’s stricter standards.

“If Oregon loses that, it will be devastating to our ability to advance on clean air, to advance on our climate goals,” she said.

Now, she has called on the Oregon Legislature and on Gov. Tina Kotek to continue efforts to adopt the stricter standards.

Or Kotek could follow her predecessor’s footsteps by issuing an executive order to continue the policies that were in place before Trump took office, Peveto said.

“I wish I could see that kind of conviction right now in our state,” she said. “I think it’s a huge opportunity in a state that voted very differently and very much for the values of environment and humanity and justice in our state to take that action today and to stand even stronger when we’re seeing that kind of antics in Congress.”

The U.S. Constitution gives Congress the power to regulate commerce, however, and it’s not clear if states have the right to challenge the Senate’s vote – which did not follow typical lawmaking processes.

Department of Environmental Quality Director Leah Feldon called the vote to revoke California’s Section 177 waiver disappointing. She said the state agency will monitor the situation and evaluate all possible effects.

“This action puts decades of progress at risk and makes it harder for states like Oregon to tackle climate change and protect our air and our communities,” she said in a statement.“ DEQ has long relied on these waivers to adopt strong air quality regulations.”

A day after the U.S. Senate vote, Kotek, along with the 10 state governors that adopted California’s stricter clean truck rules, launched a new multi-state effort that would continue the commitment to electrifying the transportation sector.

“The federal government and Congress are putting polluters over people and creating needless chaos for consumers and the market, but our commitment to safeguarding Americans’ fundamental right to clean air is resolute,” they said in a joint statement. “We will continue collaborating as states and leveraging our longstanding authority under the Clean Air Act, including through state programs that keep communities safe from pollution, create good-paying jobs, increase consumer choice, and help Americans access cleaner and more affordable cars. As we consider next steps for our clean vehicle programs, our states will engage stakeholders and industry to provide the regulatory certainty needed while redoubling our efforts to build a cleaner and healthier future.”

Oregon’s Clean Truck Rules were originally set to go into effect in January, but instead the state adopted temporary rules to give the Department of Environmental Quality more time to develop permanent rules by later this year.

The rules would have required truck engine manufacturers to meet tougher emission standards for particulate matter and for nitrogen oxides pollution, which is the result of the combustion of fossil fuels in vehicles. Rules would also have required a percentage of large pick-up trucks, buses and 18-wheelers sold by manufacturers to be zero emissions electric starting in 2025. Those sale percentages would increase over time, depending on the class size of the vehicle, but manufacturers would not have to ban the sale of diesel vehicles.

For now, DEQ has opted to delay enforcement of zero emission sales rules until 2027.