The Environmental Protection Agency on Monday took steps to challenge California’s decades-old right to set its own air pollution rules, setting up a showdown between the federal government and a state that has emerged as a bulwark against the Trump administration’s policies.

The E.P.A. statement was part of the agency’s widely expected decision to reconsider, and most likely roll back, Obama-era rules requiring automakers to hit ambitious emissions and mileage standards by 2025. The statement, though, was notable for the forcefulness of its language suggesting that the Trump administration would take on California’s authority to set its own rules.

Scott Pruitt, the E.P.A. administrator, signaled that he aimed to make California fall in line. The Obama administration, he said, “made assumptions about the standards that didn’t comport with reality, and set the standards too high.” California’s history of setting its own emissions rules “doesn’t mean that one state can dictate standards for the rest of the country,” Mr. Pruitt said.

A rollback of the rules, which are designed to cut back on emissions of greenhouse gases, would reverse one of the single biggest steps any government has taken to tackle climate change. California has said it will stick with the tougher, Obama-era regulations, a decision that could effectively split the United States into two auto markets: one requiring cars to be more efficient and less polluting than the other.