Last week U.S. EPA Administrator Scott Pruitt said California’s right to regulate its own auto emissions was not being reviewed – sending a sigh of relief for some, uncertainty for others, but the state intends to hold the line come what may.

California is defiantly moving ahead on rules the Trump administration has put the brakes on, but unclear is whether the road ahead will be rocky or smooth.

At issue is the state’s waiver permitted under the 1970 Clean Air Act enabling it to apply to the EPA to set its own emissions standards. In his confirmation hearing earlier this year, Pruitt rattled a saber that in addition to federal rules for 2022-2025 being brought under review, California’s legal precedent may also be challenged.

At the behest of the auto industry lobby, the EPA has since put the federal rules on hold until a new determination can be made next year. These call for “54” mpg over the “36” mpg of today – on 2017 window stickers the average is actually around 26 mpg, and by 2025, if rules remain unchanged, it may rise to high 30s.

But last Thursday, Pruitt seemed to back down against California whose rules are in part or whole mirrored by 13 other states.

“Currently the waiver is not under review,” Pruitt said in response to a pointed question by Rep. Ken Calvert (R-Corona), who observed the state has long received bipartisan support for its right to set its own rules – which Pruitt in turn acknowledged.

“This has been something that has been granted going back to the beginning of the Clean Air Act because of the leadership California demonstrated,” Priutt said, and in doing so he stoked optimism that the Trump administration would leave California alone.

Whether the Trump EPA will or can afford to do so remains an open question among environmental advocates hoping for the best, but preparing for the worst.

“I’d characterize it as a temporary reprieve,” said the Natural Resource Defense Council’s Roland Hwang, director of its energy and transportation program. “Note that Pruitt said ‘currently’ the waiver is not under review. This reflects the reality that there is neither a technical nor a legal basis for withdrawing the waiver. It looks like a clear loser in court.”

It also appears the automakers for now are OK with the decision too.

“We’ve not asked for the agency to review that decision,” said Daniel Gage, a spokesman for the Alliance of Automobile Manufacturers to the LA Times.

Bill Magavern, policy director for Los Angeles-based Coalition for Clean Air, told the Times Pruitt’s words were “a rare bit of good news out of the Trump administration,” but really, time will tell.

“It’s good news, but it’s only good news if they don’t end up changing their minds,” said Kate Larsen, an Oakland-based director at the Rhodium Group, which tracks climate policies to the Times.

Meanwhile, in California, it’s full steam ahead – or should we say they are fully charged? In any case, in March California’s Air Resources Board moved forward in unanimously approving its clean air laws.

It also reaffirmed its policy calling for more than one-million plug-in hybrids and/or electric vehicles on the road by 2025 – a four-fold increase from today. The U.S. EPA has no such corollary to the California ZEV rules, and the law in question does not pertain to these.

They’ve been a reality however, and automakers have historically built cars for California rules, and so it’s believed and/or hoped they will still do so even if the EPA bows to automakers to reverse course on federal Corporate Average Fuel Economy (CAFE) rules.

“They will certainly try, otherwise why would they bother redoing the final determination?” asked Hwang of the Trump EPA attempting to weaken federal rules. “This is a very polluter friendly administration and probably the worst EPA administrator ever. During Pruitt’s days as Oklahoma Attorney General, he essentially had polluters write his policies. The auto industry has carte blanche to rewrite these rules. The only thing stopping them from doing so is perhaps their own better judgment that they ultimately can’t win a battle with California, environmental groups and the courts.”

Uncertain though is how aggressively they’d want to cut back, because being global companies, they still are developing vehicles for the rest of the world which is following increasingly strict rules.

And then there is also California, standing as a bastion for policies the Obama administration embraced, and which the new one wants to shy away from.

Presently, 13 states have in place California’s LEV III rules – about 40 percent of the nation – but only 10 states including California are following the aforementioned ZEV (Zero Emission Vehicle) rules.

That’s still a big market, so while Pruitt says “currently” California’s waiver is not being reviewed, if it is never reviewed and ultimately challenged, even if federal CAFE rules are rolled back, automakers will still be forced to develop cars for that large market.

This in turn sets up a potential legal showdown, and talk by California and pro-environment supporters has been openly made for the past few months of willingness to aggressively engage in a legal battle.

“There are only two routes to block California, that’s Congress and the courts,” said Hwang. “Taking away California’s authority in either venue is extremely tough, but they may not stop them from trying like in the past.”

This, he says is the case despite other pundits offering that the EPA administrator may easily revoke California’s waiver with a “stroke of a pen.”

Because of “the rule of law,” and California’s longstanding precedent, even if a fast move was tried by the EPA, it would not be so easily upheld in court, contends Hwang.

So, for now, while the jury is out on federal resolve to clean tailpipe emissions and raise mpg for passenger vehicles from 2022-2025, California stands as the holdout.

It actually already was, as even the present CAFE rules written under the Obama administration allowed workarounds to manufacturers so as not to force them to build plug-in vehicles if they do not want to. Former National Highway Transportation Safety Administrator David Strickland said in 2013 only 1-3 percent of vehicles would need to be plug-in electrified to meet CAFE standards by 2025 as they stood under Obama.

“Our analysis at NHTSA projects that the automakers can meet these standards largely through advantages in internal combustion engines,” said Strickland in 2013. “We project that the automakers will only need to produce about 1 to 3 percent of electric vehicles from plug-ins to meet the 2025 standards.”

Thus, with the prospect of an even weaker standard, and no ZEV rules on the federal level, California’s role in pushing the EV agenda becomes all the more clear.

“California will continue to play its role to lead the nation, as it has for over 50 years, on clean cars,” said Hwang. “CARB has already stated its intention to develop the next phase of its rules beyond 2025.”

And if the political winds change later, there would be a fight.

“If Priutt and automakers want to take a run at the waiver,” said Hwang, “it will be a long, hard legal battle, one that will back them up to lead time issues and the next administration.”