President Donald Trump and California’s Democratic Gov. Gavin Newsom escalated their confrontation on Monday over the handling of protests in Los Angeles over the federal government’s immigration crackdown.

Trump, arriving back at the White House after spending the weekend at Camp David, told reporters he would arrest Newsom if he were “border czar” Tom Homan — hours after Homan said there had been no discussions about arresting Newsom.

“I would do it if I were Tom. I think it’s great,” Trump told reporters.

Newsom quickly fired back.

“The President of the United States just called for the arrest of a sitting Governor,” Newsom posted on Instagram along with a video of Trump’s comments. “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.”

Just hours before Trump spoke, Homan himself had pushed back on the idea he was going to arrest Newsom and Los Angeles Mayor Karen Bass, both Democrats.

In an interview with NBC News over the weekend, Homan had not ruled out the possibility — prompting Newsom to respond: “He knows where to find me.”

Homan on Monday morning, during an interview on Fox, commented further on his remarks to NBC.

“The reporter asked about, ‘Could Governor, Governor Newsom, or Mayor Bass, be arrested? I said, ‘Well, no one’s above the law, if they cross the line and commit a crime. Absolutely they can.’ So there was no discussion about arresting Newsom,” he said.

“I’ve said it many times, You can protest, you got your First Amendment rights, but when you cross that line, you put hands on an ICE officer, or you destroy property, or ICE says that you’re impeding law enforcement … That’s a crime, and that the Trump administration is not going to tolerate. You cross that line we’re gonna see prosecution in the Department of Justice,” Homan said.

Trump on Monday also doubled down on his decision over the weekend to deploy the National Guard to California, over objections from Gov. Newsom.

This is a developing story. Please check back for updates.