A legal battle has already begun.
While President Trump's tweets are mostly garbled rants appealing to his base, occasionally actual information creeps out. In this case, President Trump was perfectly clear when he said, "The Trump Administration is revoking California's Federal Waiver on emissions in order to produce far less expensive cars for the consumer, while at the same time making the cars substantially SAFER."
The crux of the matter here is that California enforces its own emissions standards at a more stringent level than the federal standards. Trump is going to strip California of its right to do that, and at the same time roll back federal fuel economy standards.
The announcement will to lead into an immense legal battle between the feds and California that will leave the automotive industry in turmoil. Automakers will either have to bet big on one outcome or take on the huge additional expense of developing multiple technologies so any possible outcome can be met. It'll make automotive manufacturing commitments in North America uncertain, and if there's one thing that'll hurt an industry, it's uncertainty.
Right now, automakers simply have to meet the highest standard to sell cars in every state. This act of "deregulation" may sound good on the surface for automakers, but the automakers know that splitting the US market is a bad idea. And when huge corporations oppose deregulation, that's a huge sign that it's a dangerous thing.
In June 17, automakers wrote to Trump asking for reassurance on mileage standards and explaining that what they want is one national standard for vehicle fuel economy that California can support in order to avoid the oncoming war. That would require a deal being made. However, Trump hasn't been able to do that yet.