In a televised address on Tuesday night, California Governor Gavin Newsom accused President Donald Trump of a "brazen abuse of power".
The Trump administration's immigration enforcement efforts were terrorising his state's immigrant neighbourhoods, he said.
"California may be first, but it clearly will not end here," he warned. "Democracy is under assault before our eyes."
Newsom's speech was the latest salvo between the US president and the Democrat who heads America's most populous state.
The clash between the two leaders presents a formidable challenge to the 57-year-old governor.
