The petulant adolescent in the White House — who has replaced most of the adults around him with raging sycophants and has demoted his chief of staff, John Kelly, to lapdog — lacks adequate supervision.

Before, he was merely petty and vindictive. He’d tweet nasty things about people he wanted to humiliate, such as former San Francisco 49ers quarterback Colin Kaepernick.

Now his vindictiveness has turned cruel. After smearing FBI Deputy Director Andrew McCabe with unfounded allegations that he lied to investigators, the new Donald Trump made sure McCabe was fired just days before he would have been eligible for a pension after more than 21 years of service.

Before, he was merely xenophobic. He’d call Mexicans murderers and rapists.

Now Trump’s xenophobia has turned belligerent. He’s sending thousands of National Guard troops to the Mexican border, even though illegal border crossings are at a record low.

And he’s starting a trade war against China.

China has been expropriating American intellectual property for years. But Trump isn’t even trying to negotiate a way out of this jam or build a coalition of other trading partners to pressure China. He’s just upping the ante — and, not incidentally, causing the stock market to go nuts.

But the most dangerous thing about the new Trump is his increased attacks on American democracy itself.

Start with a free press. Before, he just threw rhetorical bombshells at the Washington Post, CNN and other outlets that criticized him. Now he’s trying to penalize them financially while bestowing benefits on outlets that praise him.

Last week, Trump demanded that Amazon, the corporation headed by the man who owns the Washington Post, pay higher postal rates and more taxes, and that the Post should register as Amazon’s lobbyist. Amazon stock wilted under the attack.

They’re absurd charges. Amazon collects and pays state sales taxes on its products, and the U.S. Postal Service is losing money because of the decline in first-class mail, not package deliveries.

Presumably, Amazon can take care of itself. Trump’s attack was intended as a warning to other companies with media connections that they’d better not mess with him

Trump is trying to hurt CNN, too. The day after the Justice Department moved to block AT&T’s purchase of Time Warner, parent of CNN, he said the deal wasn’t “good for the country.” Few missed the connection.

Meanwhile, he’s praising Trump-adoring Sinclair Broadcasting, signaling to the FCC that it should approve Sinclair’s pending $3.9 billion purchase of Tribune Media’s TV stations.

We’re entering a new and more dangerous phase of Trump’s “divide and conquer” strategy, splitting the nation into warring camps — with him as the most divisive issue.

Even Trump’s tweets have become more brazenly divisive. This month he called his predecessor “Cheatin’ Obama.” When was the last time you heard a president of the United States disparage another president?

Trump is more determined than ever to convince supporters that Special Counsel Robert Mueller is in cahoots with Democrats and the FBI to unseat him. This might give him some protection if Trump decides to fire Mueller, or if Mueller’s investigation turns up evidence that Trump collaborated with Russia to win the election, and Congress moves to impeach him.

“Try to impeach him, just try it,” Roger Stone, Trump’s former campaign adviser, warned last summer. “You will have a spasm of violence in this country, an insurrection like you’ve never seen.”

But Trump’s strategy might just as easily extend beyond Mueller. What happens if in 2020 a rival candidate accumulates more electoral votes, but Trump accuses him or her of cheating and refuses to step down?

“He’s now president for life,” Trump recently said of Chinese President Xi Jinping, adding: “Maybe we’ll have to give that a shot someday.”

Some thought Trump was joking. I’m not so sure.

Democracies require leaders who understand that their primary responsibility is to protect the institutions and processes democracy depends on. The new Trump seems intent on maintaining his power, whatever it takes.

Democracies also require enough social trust that citizens regard those they disagree with as being worthy of an equal say, so they’ll accept political outcomes they dislike. The new Trump is destroying that trust.

Trump untethered isn’t just a more petty, vindictive and belligerent version of his former self. He’s also more willing to sacrifice American democracy to his own ends. Which makes him more dangerous than ever.

© 2018 Robert Reich

Robert Reich, a former U.S. secretary of labor, is professor of public policy at UC Berkeley. To comment, submit your letter to the editor at SFChronicle.com/letters.