It’s looking more and more likely that Secretary of Defense James Mattis’s days in the Trump administration are numbered. If he leaves, experts say it could make an already shambolic presidency even more chaotic.

The greatest sign of Mattis’s ouster is an interview President Donald Trump gave to CBS’s “60 Minutes” on Sunday, in which he insulted his defense chief and left the door open for the secretary’s departure.

“I think he’s sort of a Democrat, if you wanna know the truth,” Trump told Lesley Stahl. “But General Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That’s Washington.”

Mattis, a retired four-star Marine Corps general, is considered by many outsiders to be one of the president’s best Cabinet members in part because he’s seen as a strong check on Trump’s worst national security impulses.

But Trump’s comments, added to months of reports that have detailed Trump’s displeasure with Mattis, indicate the the secretary is on the outs. It’s unclear as of now, though, what particularly has angered Trump.

If Mattis goes — after the midterms, as many reported — Trump would be left with few if any moderating influences on his national security team.

Mattis’s empty seat would leave National Security Adviser John Bolton and Secretary of State Mike Pompeo to offer more hawkish advice on North Korea, Iran, and other issues with minimal pushback from the Pentagon.

Which means the last “adult in the room” would cede control to an increasingly aggressive president and the advisers who enable him.

Mattis works to keep Trump at bay. He’s failing.

Mattis — along with his now-ousted colleagues National Security Adviser H.R. McMaster and Secretary of State Rex Tillerson — was frequently labeled ones of the few adults in the room whose military and strategic expertise helped him gain Trump’s trust and moderate the president’s most dangerous foreign policy impulses.

For instance, Mattis reportedly stopped Trump from ordering the assassination of Syrian President Bashar al-Assad in April 2017, a move that would have escalated Syria’s brutal civil war and brought the US much deeper into the conflict. He also pushed Trump to stick to a diplomacy-first approach to North Korea rather than defaulting immediately to military options. And according to Trump himself, Mattis convinced him that torture is a bad idea.

Yet despite those successes, Mattis failed to bring Trump around to his position on several key foreign policy issues. Perhaps the biggest one was the Iran nuclear deal.

Mattis, along with Tillerson, opposed pulling the US out of the Iran deal — something Trump had promised to do on the campaign trail. They argued that Iran, despite its support for terrorism in the region, had not violated the terms of the deal by working toward a nuclear weapon. And for a while, they were successful, forcing Trump to recertify the deal every three months.

However, eventually Trump’s will won out, and in May, the president announced he was officially pulling the US out of the nuclear accord.

Mattis also opposed moving the US Embassy in Israel to Jerusalem, creating a Space Force, and starting a trade war with America’s European allies. But he failed to persuade Trump to see things his way on each of those issues, despite his once-good rapport with the president.

Perhaps even worse, Trump also kept Mattis out of the loop on other major military announcements, including halting military exercises with South Korea and banning transgender troops from the military. (Trump announced the latter while Mattis was on vacation.)

Yet despite his limited ability to sway the president, Mattis chose to stay in the administration — outlasting both McMaster and Tillerson.

Mattis is the anti-Trump. That finally caught up to him.

Despite the widespread relief among leaders from both parties upon Mattis’s appointment, it was always clear that Mattis and Trump diverged in their foreign policy views and that a future clash was almost a certainty.

Trump famously believes in an “America First” approach, in which US relationships with foreign countries are determined not by traditional alliances and friendships but rather by what America gets out each relationship — often measured in dollar amounts. Mattis, meanwhile, believes in America’s role as the protector of global order and in maintaining strong economic and military relations with traditional US allies.

Ultimately, Trump wants to disrupt world affairs while Mattis wants to maintain the status quo. So even though they began their time together speaking several times a day on the phone, their fundamental disagreements seem to have pushed them further and further apart as time went on — and now they rarely talk.

Further, Trump has reportedly stopped calling Mattis “Mad Dog” — a Marine Corps nickname Mattis didn’t like in the first place — in recent weeks and has instead, according to Politico, begun calling him “Moderate Dog” behind closed doors. That may have led Trump to label him a “Democrat” — a label he’s also given to Deputy Attorney General Rod Rosenstein, the overseer of Robert Mueller’s Russia probe.

Journalist Bob Woodward’s bombshell September book Fear, exposing the inner workings of the Trump administration’s two years in office, probably didn’t help their relationship either.

According to Woodward, Mattis thought Trump acted and understood things like “a fifth- or sixth-grader,” and openly remarked to friends that “Secretaries of defense don’t always get to choose the president they work for.” (Mattis has denied saying these things.)

This may have contributed to Trump’s anger and may eventually force Mattis out. Others, though, would prefer he stay.

“I’d rather have Jim Mattis in the room than not in the room,” former CIA and Pentagon chief Leon Panetta told me in March. Should he go, “his legacy will be he fought the good battle. But because of the nature of the president, it was one he couldn’t win.”