In a hotel meeting room two blocks from the beach, the California Coastal Commission might as well have written “S.O.S.” in the sand.

“We are under complete assault,” said Susan Lowenberg, a member of the coastal-preservation commission, which was born in the 1970s from the same movement that gave rise to Earth Day and the U.S. Environmental Protection Agency. “We need help.”

President Trump has publicly castigated the state commission, vowing not to “let them get away with their antics.” Elon Musk has said it “should not even exist as an organization.” Mr. Trump’s administration has threatened to withhold federal aid for the Los Angeles wildfires unless the state defunds the agency.

The attacks are surprisingly bipartisan: Democratic state lawmakers are trying to weaken the commission’s authority over housing development. And Gov. Gavin Newsom, a Democrat, has said the commission has too much power, and delivered a stinging rebuke in January.

The California coast stretches 840 miles, from mossy redwood forests in the north to cliff-side waterfalls on the Central Coast and palm tree-studded beaches in the south. Millions of visitors are drawn each year to its kaleidoscope of landscapes and pristine shoreline.