A group of 17 major automakers have co-signed a letter to President Donald Trump asking him to not go through with his plan to significantly lower emissions standards in the US, according to reports from The New York Times and Reuters. The letter, delivered today, reportedly calls Trump’s plan “untenable.”

Trump has been pushing to lower emissions standards for over a year now, and while it might seem counterintuitive that major car manufacturers would be against that, this letter makes crystal clear that are. The issue is that California has much more stringent standards and is leading 17 states in a suit against the EPA over emissions as well.

That’s the situation that had led to this letter, just ahead of what was meant to be rollout of the new rules in the Rose Garden at The White House. These car makers don’t want two radically different sets of rules in the US. Instead, they are backing a compromise.

In fact, the same automakers have also reportedly sent a letter to California Governor Gavin Newsom, reportedly arguing that they want a “final rule supported by all parties — including California” that is “midway” between Obama era regulations and what Trump is proposing.

The Times suggests that Trump could respond to the letter in an extreme way, reporting that “some officials have said that they fear auto industry criticism of Mr. Trump’s rollback could lead the president to retaliate by imposing tariffs on auto imports.” Were that to happen, it could significantly increase the chances that the ongoing trade war could push the US economy into a recession. Trump’s trade wars already threaten the industry.

The automakers who reportedly signed the letter to Trump include Ford, GM, Toyota, Volvo, BMW, and Volkswagen. Fiat Chrysler reportedly has not, which the Times says is because it is more supportive of Trump’s plan — but also, the company probably also has other things on to worry about right now after its merger with Renault collapsed.

As Vox has reported, car makers are much more worried about “uncertainty” than they are about the precise details of the rules. They just need to know what the rules will be so they can build cars that target them. But with this administration, hoping for predictability in issues of trade is probably asking too much.

Update: reached for comment, Newsom’s office provided the following statement: