President Donald Trump is reportedly thinking about replacing James Mattis as defense secretary.

According to Politico, Trump has also replaced Mattis' "Mad Dog" nickname with a new one: "Moderate Dog."

Mattis is said to be a moderating force in the administration, slow-walking Trump's directives and even ignoring the more incendiary ones.

For President Donald Trump, the luster seems to have worn off Defense Secretary James Mattis.

Trump heralded the retired Marine general when he was picked for the top job at the Pentagon at the end of 2016.

Trump has praised him as "a true General's General" and has told donors, "The guy never loses a battle, never loses."

Trump has frequently referred to Mattis as "Mad Dog" — though Mattis has expressed an aversion to that nickname, telling senators during his confirmation hearings that it was given to him by the press and that "perhaps they didn't get it quite right."

Friends of Mattis told CNN last year that people who know Mattis do not call him "Mad Dog."

But reports have continued to emerge suggesting that Mattis is a moderating force in the administration, slow-walking Trump's directives and even ignoring the more incendiary ones (and, on at least one occasion, defusing confrontations between administration officials).

According to a Politico report published Wednesday, Trump has picked up on this dynamic, and it has affected how he views the defense secretary.

Behind closed doors, Trump has reportedly started calling the revered general "Moderate Dog," and he is said to have been considering replacing Mattis for months.

Trump and Mattis at a Cabinet meeting at the White House in March 2017. REUTERS/Jonathan Ernst

Mattis was not Trump's first choice for the job. But one of Trump's reported top choices, retired Army Gen. Jack Keane, and other national-security figures, like the late Sen. John McCain, spoke highly of Mattis, who headed US Central Command before retiring in 2013.

A former senior White House official told NBC News this summer that Trump and Mattis "don't really see eye to eye."

That NBC News report described Mattis as having been blindsided by Trump's decisions to withdraw from the Iran nuclear deal, pause military exercises with South Korea, and create a sixth military branch focused on space.

While rumors about Mattis' fate have circulated for some time, the Pentagon chief is now expected to resign or be dismissed after midterm elections in November, according to Politico.

The White House is said to have a shortlist of potential replacements.

Two people reportedly on the list — Sens. Lindsey Graham of South Carolina and Tom Cotton of Arkansas, both Republicans who are up for reelection in 2020 — have indicated they aren't interested, Politico said.

Others under consideration include Keane and former Sen. Jim Talent of Missouri, The Washington Post reported last week.

Trump and Mattis have both denied reports of a growing rift between them. Earlier this month, when asked about Mattis' future after reports that he had made disparaging remarks about the president, Trump said Mattis would remain.

"We're very happy with him," Trump said. "We're having a lot of victories."