SACRAMENTO — The Trump administration may appear to control climate policy in Washington, but the nation’s most dynamic environmental regulator is here in California.

Mary D. Nichols, California’s electric-car-driving, hoodie-wearing, 72-year-old air quality regulator, is pressing ahead with a far-reaching agenda of environmental and climate actions. She says she will not let the Trump administration stand in her way.

As chairwoman of the California Air Resources Board, or CARB, Ms. Nichols is the de facto enforcer of the single biggest step the United States has taken to combat the effects of climate change: standards adopted under the Obama administration that mandate a deep cut in emissions from the 190 million passenger cars on America’s roads. Together, those vehicles regularly emit more earth-warming gases than the country’s power plants.

At the request of the major automakers, the Environmental Protection Agency officially opened a review of those standards last month. The move was seen as the prelude to a loosening of those targets, which require manufacturers to nearly double the average fuel economy of new cars and light trucks by 2025.