Trump said it would be 'great' to arrest Newsom. Their stormy relationship and the politics at play.

President Donald Trump and California Democratic Gov. Gavin Newsom are engaged in a bitter fight over the handling of protests against Immigration and Customs Enforcement continue in Los Angeles -- and both also are framing their confrontation in familiar, sharp political terms.

On Monday, the president said it would be a "great thing" if border czar Tom Homan arrested Newsom; in response, the California governor fired back that the comment is an "unmistakable step toward authoritarianism."

In a sign of how much politics is driving the confrontation, Trump, when asked on Monday afternoon by ABC News what crime Newsom has committed to warrant his arrest, Trump said the governor's "primary crime is running for governor because he's done such a bad job." Newsom responded on X, "Donald Trump admits he will arrest a sitting governor simply because he ran for office."

With the protests, Trump, who has characterized them as "violent, insurrectionist mobs" and "Gavin Newscum inspired Riots," has deployed National Guard members to Los Angeles. Newsom has asked the administration to rescind the deployment and said Monday that he is suing the Trump administration, claiming Trump illegally federalized the National Guard.

But Trump is not only criticizing the protesters -- he is blasting the Democrats leading the state and the city, calling them failures.

"The very incompetent 'Governor,' Gavin Newscum, and 'Mayor,' Karen Bass, should be saying, 'THANK YOU, PRESIDENT TRUMP, YOU ARE SO WONDERFUL. WE WOULD BE NOTHING WITHOUT YOU, SIR,'" Trump wrote on his social media platform on Monday. "Instead, they choose to lie to the People of California and America by saying that we weren't needed, and that these are 'peaceful protests.'"

Trump has long been critical of the leadership in most Democratic-run states, often focusing his ire on California.

The situation, separately, gives Trump the chance to take high-profile action on immigration enforcement -- a key issue for the president during his 2024 campaign and one that has remained a priority during the first few months of his administration. A recent poll from Marquette Law School taken in early to mid-May found that Trump had positive or around even job approval on border security and immigration.

Newsom, for his part, has explicitly accused the White House of exacerbating the situation for political gain.

"They want a spectacle. They want the violence," he said in an email to supporters sent through his political action committee on Sunday night. "They think this is good for them politically." Since then, he's posted a blizzard of attacks on Trump via social media.

The White House responded to an ABC News request for comment late Monday afternoon.

“Gavin Newsom’s feckless leadership is directly responsible for the lawless riots and violent attacks on law enforcement in Los Angeles. Instead of writing fundraising emails meant to score political points with his left-wing base, Newsom should focus on protecting Americans by restoring law and order to his state," Abigail Jackson, a White House spokeswoman, said.

Newsom and Trump have long been at odds, although the two had a brief detente in their relationship in the past few months.

In the aftermath of wildfires in January that devastated the Los Angeles region, Trump visited the city toward the end of the month and was greeted on the tarmac by Newsom with several handshakes and an embrace; Newsom also met with Trump in Washington in early February, and told CNN afterwards, "I have just all the confidence in the world that it's going to be a strong partnership moving forward."

But Newsom, around that time, also approved $50 million for funds that could be used in legal battles against the federal government.

And Newsom grew more critical of Trump in the months afterward -- attacking the president's tariff policy in an ad that aired on Fox News where he said the "tariffs punish families."

The Trump administration has appeared to direct punishment at California as well. Earlier this month, Trump vowed to impose "large scale fines" on California after a transgender teen competed in a California state final competition in track and field. Last week, the Trump administration signaled that it would cut federal funding for a high-speed rail project in the state.

Newsom, separately, has begun to build a national profile amid speculation that he could run for president in 2028, which included stoking more speculation through a buzzy podcast launch in March. Newsom is term-limited and cannot run for governor in 2026.

While the Los Angeles situation is tied to Newsom's current work as governor and not to any current or future campaign, it puts him back in the national spotlight and at the center of one of the nation's highest-profile political issues, standing up to Trump.

Newsom referenced what he framed as the national stakes in his response to Trump's comments on his potential arrest: "This is a day I hoped I would never see in America… this is a line we cannot cross as a nation."

ABC News' Molly Nagle and Michelle Stoddart contributed to this report.