Long before President Trump declared that his defense secretary, whom he had previously praised as "the real deal", was actually "kind of a Democrat", National Security Advisor John Bolton and his deputy, Mira Ricardel, had launched a "whisper campaign" within the West Wing to try to oust the longtime general, eventually succeeding in cutting him out of presidential briefings due to the view that Mattis wasn't "ideologically aligned" with the administration, according to Foreign Policy.

According to two anonymous West Wing sources, Bolton and Ricardel are "trying to build a sense that [Mattis] is done for." "They have their knives out," one official added. However, the campaign to oust Mattis has been met with resistance by lawmakers and some inside the administration, who believe Mattis is one of the few members of Trump's cabinet who enjoys genuine bipartisan respect and support.

Bolton has successfully managed to cut down on the number of National Security Council meetings, ensuring that he is the closest to Trump on issues involving national security - where Bolton is known for his fervent interventionism and continued support of the Iraq war. Though one source aligned with Bolton said the idea that the NSC isn't meeting regularly enough is "100% Mattis spin."

One Trump administration official noted, "Mira and Bolton are the only ones who benefit if Secretary Mattis leaves." The secretary is "highly regarded" within the cabinet and by lawmakers on both sides of the aisle, the official said. The question of Mattis’s departure is closely watched in Washington, in part because he’s viewed as an experienced professional with a steady hand in an administration often plagued by turmoil. In countries that have had disputes with the Trump administration - on Iran or NATO, for example - knowing that Mattis has a voice in decision-making has been reassuring. The former official said the irregularity of National Security Council meetings—in which the president gets assessments and opinions from an array of officials, including the defense secretary—is a point of frustration for the Pentagon. "What that means is that the president is not regularly hearing in any organized and disciplined fashion from the full range of his advisors," the former official said. "He is getting [the meetings] one-off and ad hoc, and often controlled by Bolton."

To be sure, NSC spokesman Garrett Marquis also denied the allegations.

"Ambassador Bolton is working closely with Secretary Mattis to implement the President’s agenda," Marquis said. "Any indication otherwise is flatly wrong."

While rumors that the relationship between President Trump and Mattis had deteriorated had been circulating for months as Trump cut him out of major policy decisions, including the decision to cancel planned joint military exercises with South Korea, the drumbeat of negative stories intensified last month when the New York Times reported that Mattis could be pushed out as soon as the midterms were over. A few weeks later, Trump declared that Mattis was "sort of a Democrat" during an interview earlier this month, adding that he "may leave" the administration.

But sources close to Mattis reportedly told Foreign Policy that Mattis likely wouldn't resign. He would need to be pushed out, with one adding that Mattis "wouldn't walk away." "They may force him out, but he won't quit," one said.

But Mattis’s allies insist the defense secretary will not resign. If Trump wants him out—and that’s still a big “if,” they say—the president will have to fire him. "I’ve known him for a long time, and he will not walk away," said Michèle Flournoy, a former undersecretary of defense for policy who was Mattis’s first choice for deputy defense secretary (she ultimately turned him down). "They may force him out, but he won’t quit," Flournoy said. "He feels duty-bound to be there." A former senior military commander echoed the sentiment. "He’s not the kind of guy who takes a knee."

Yet another source from within the administration said Mattis's departure is a virtual certainty, claiming that the relationship "is just not working anymore." Though several foreign policy insiders quoted by FP praised Mattis's ability to preserve America's defensive alliances amid the at times chaotic policy proclamations of President Trump, who has attacked NATO and demanded that America's allies pay for their protection from the US.

Soon after Nov. 6, "he will either be gone, or he will make it clear that he is leaving," said a second administration official. "The reality is it’s not working anymore, and this is the ideal time to make a break." The administration has had its share of high-level departures, some smooth and some ugly. Mattis has many supporters in Washington, among both Republicans and Democrats, making it politically risky for Trump to move against him in a way that might be seen as disrespectful. "Secretary Mattis brings great respect and gravitas," said Tom Spoehr, the director of the Center for National Defense at the conservative Heritage Foundation. Flournoy noted that Mattis has managed to keep America’s defense alliances on track “despite the uncertainty and unpredictability of broader U.S. policy.” Inside the department as well, "he has been a real source of stability," she added.

Assuming Mattis does leave, Trump might struggle to confirm an "ideologically aligned" replacement, as Bolton's confirmation was a narrow win and Republicans like Rand Paul who oppose Bolton's neoconnishness might agree to join with Democrats to try and sink whomever Bolton decides to pull for. And there's also the question of how the US's NATO allies would react to Mattis's ouster, as some believe that he has been the key figure in the administration who has kept these military relationships intact amid Trump's persistent bashing of the Cold War alliance.