While Mercedes-Benz has not confirmed its plans to join those automakers, it wouldn't be entirely surprising if the company did so. According to The New York Times, California and 13 other states plan to enforce their current, stricter emissions regulations. Those states may also sue the Trump administration. Rather than wait for the legal drama to unfold or navigate the discrepancies between state and federal emissions requirements, the automakers have voluntarily agreed to follow California's rules.

A sixth automaker could join the pact soon, too. While The New York Times did not confirm which, it is believed either Toyota, Fiat Chrysler or GM will pledge to follow stricter emissions standards for at least the next four years. The six automakers would account for more than 40 percent of all cars sold in the US, and together, they could significantly weaken Trump's emissions standards rollbacks.