Though the E.P.A. approved California’s greenhouse gas standards in 2013, the agency now argues that they impermissibly conflict with another federal law, the Energy Policy Conservation Act. That law, passed in 1975 in response to the energy crisis, gives the Transportation Department the sole power to set standards “relating to fuel economy.” Because auto companies comply with California’s greenhouse gas standards primarily by improving fuel efficiency, the E.P.A. now says in an about-face that the standards are pre-empted by those Transportation Department regulations. On the same theory, California’s program to promote electric and hybrid vehicles would also be eliminated.

If the Trump administration succeeds in scuttling the federal standards and neutralizing California, it could be the most significant setback for American progress on climate change so far under President Trump. Worse, this disastrous policy could continue well beyond his tenure. Future presidents could not simply undo the decision, if courts rule that the Trump administration’s interpretation of the law is categorically correct. In that case only Congress could restore California’s authority.

Here is what is at stake: The Obama-era standards that President Trump seeks to scrap would gradually raise average fuel economy under test conditions from about 37 miles per gallon in 2020 to 50 miles per gallon in 2025 — a near-doubling of fuel efficiency and halving of greenhouse gas emissions since 2010. This would reduce American oil consumption by 1.2 billion barrels, cut half a billion metric tons of carbon pollution and save consumers billions of dollars in fuel costs, according to the E.P. A’s own recent estimates.

For over 50 years, California has been the undisputed leader among the states in addressing air pollution and its harms. In the 1950s and 1960s, the state had some of the dirtiest air in the world, in large part because of vehicular pollution. California required auto companies to install the first pollution control technology in 1961, and set the nation’s first tailpipe standards for emissions in 1966. In 1967, Governor Ronald Reagan signed legislation creating the powerful California Air Resources Board to regulate air pollution statewide, including by strictly curbing vehicle emissions.

Congress recognized California’s leadership when it passed the Air Quality Act of 1967, allowing the state to set its own stricter-than federal standards for vehicles. Congress cemented this approach in the 1970 Clean Air Act, and reinforced it again in 1977, letting other states adopt California’s standards.