President Donald Trump threatened Monday to arrest Gov. Gavin Newsom, a striking example of Trump attacking a fellow elected official just as National Guard troops deployed in Los Angeles amid protests over immigration actions.

“The President of the United States just called for the arrest of a sitting Governor,” Newsom wrote on social media. “This is a day I hoped I would never see in America. I don’t care if you’re a Democrat or a Republican, this is a line we cannot cross as a nation — this is an unmistakable step toward authoritarianism.”