WASHINGTON — Rumors that President Donald Trump is considering replacing Defense Secretary Jim Mattis continue to grow, buoyed by a new report claiming the commander in chief has taken to calling the former Marine general “Moderate Dog” Mattis behind closed doors out of frustration with his measured political positions.

Mattis, a wildly popular figure in military circles, is known to many as “Mad Dog,” a nickname stemming from his frequent macho comments like: “Be polite, be professional, but have a plan to kill everybody you meet." Mattis himself has said he is not a fan of the name, calling it more of a media creation than a reflection of his personality.

But Trump has not been shy about using the “Mad Dog” nickname in public speeches, often referring to it as proof that he picked a battle-hardened and aggressive leader for the Defense Department.

A report by Politico Wednesday night said Trump is now souring on that view, believing that Mattis is more moderate than he had hoped. The new “Moderate Dog” moniker suggests that Mattis could leave or be dismissed from the top Pentagon post in coming months, as Trump looks for Cabinet members more closely aligned with his policies.

Earlier this week, when asked by reporters about his relationship with Trump, Mattis said he sees “no problem” and “it’s been the same all along.”

But when pressed if he expects to remain defense secretary through Trump’s entire first term in office, Mattis would only say that “this is not a day I’m going to go further into politics."

Mattis’ three most recent predecessors — Ash Carter, Chuck Hagel and Leon Panetta — only averaged about two years apiece in the top Pentagon role. January 2019 would mark Mattis’ second anniversary in the job.

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But his departure would cause shock waves on Capitol Hill, where many lawmakers see Mattis as a calming force within the turbulent administration.

In recent months, both Democrats and Republicans critical of Trump’s policies have said they still have faith that the defense secretary can keep military operations running smoothly and responsibly, and that Mattis continues to be a voice of reason for the impulsive president.

In his new book “Fear: Trump in the White House,” Bob Woodward chronicles several incidents of behind-the-scenes friction between Trump and Mattis (which both men have denied).

They include allegations that Trump demanded Mattis rapidly increase operations against Syrian leader Bashar al-Assad, but Mattis instead told military leaders “we’re going to be much more measured” in the campaign.

The book also alleges Mattis told associates that the president acts like “a fifth- or sixth-grader” and “secretaries of defense don’t always get to choose the president they work for.”

In addition, Mattis’ Pentagon has appeared repeatedly caught off guard by major defense announcements coming from the White House, including the creation of a new “Space Force” branch of the military, the proposed cancellation of military exercises with South Korea, and several moves related to the Iran nuclear deal.

Reporter Tara Copp contributed to this story.