I have serious doubts that this will happen, but even if the federal government were successful in forcing more polluting cars on California and the states that follow it against their will, one immediate unintended consequence of that would almost certainly be the adoption of state-level policies that avoid the emission problem altogether by accelerating the adoption of clean BEVs along with solar power.
The economics are already there and the transition is already happening, and I am quite confident that if the feds tried to force weaker pollution laws on California, Jerry Brown and the CA legislature would not hesitate to speed up the transition to a clean energy and clean transportation future.
In fact, Governor Brown has already been quite outspoken about his resolve to protect the environment, regardless of what happens in Washington.
Jerry Brown vows to fight Trump on climate: "California will launch its own damn satellite" And there is a ton of political support in California to back him up.
Bottom line: if they try this, it could backfire in a very big way.