This article originally appeared on Climate Central.

The head of the city department that drafts many of San Francisco’s greenest rules and regulations uses one word to explain her greatest fear for the environment during Donald Trump’s presidency: “preemption.”

If some of the deepest concerns of climate-focused bureaucrats from San Francisco to Massachusetts and New York come true, the Trump administration will preemptively prevent them from acting to slow global warming.

With Trump and Republicans in Congress widely expected to unite to undermine federal environmental protections, progressive states and cities are making plans to fight global warming within their borders without being helped or required to do so by the U.S. government.

“Preemption is probably the progressive cities’ worst nightmare,” said Deborah Raphael, director of San Francisco’s environment department, which has helped city lawmakers craft rules mandating everything from greener buildings to composting and recycling by residents. “It’s also the state of California’s worst nightmare.”