Donald Trump has denounced his own former chief of staff, John Kelly, as a “degenerate” and a “low life” after the former US Marine Corps general gained the backing of Kamala Harris for calling his ex-boss a fascist.

With Kelly’s intervention in effect propelling the debate over fascism firmly to the centre of the US presidential election, the Republican nominee also turned his fire on his Democratic opponent. He inaccurately accused Harris of calling him Adolf Hitler after the vice-president amplified Kelly’s comments in a televised address before endorsing them in a CNN town hall meeting.

Trump’s angry fusillade came in social media posts amid the fallout from Kelly’s comments in a New York Times interview in which he recalled the former US president repeatedly lauding Hitler’s achievements when he was in the White House.

In a separate interview with the Atlantic, Kelly described Trump lamenting that he did not have generals who were loyal in the way he believed German military commanders had been to Hitler.

Trump responded on his Truth Social platform, calling Kelly – who was his White House chief of staff for 18 months – a “degenerate…..who made up a story out of pure Trump Derangement Syndrome Hatred.

“This guy had two qualities, which don’t work well together,” he wrote. “He was tough and dumb. John Kelly is a low life.”

Kelly told the Times that Trump “fitted the general definition of a fascist” and said he would rule as a dictator if elected again.

In Wednesday’s statement, Harris – who had been issuing increasingly strident warnings on the campaign trail about Trump’s authoritarian outlook in the face of his increasingly threatening rhetoric – said the interview showed that he sought “unchecked power”.

She added that it was “deeply troubling and incredibly dangerous” that he would “invoke” Hitler. She later told the CNN moderator Anderson Cooper that she agreed that Trump was a fascist and praised Kelly for sending a “911 call” to the nation.

Trump responded with a post on X that received more than 20m views and 292,000 likes, accusing Harris of “going so far as to call me Adolf Hitler, and anything else that comes to her warped mind”, because, he claimed, polls indicated she was losing.

Trump’s campaign’s communications director, Steven Cheung, accused Harris of “dangerous rhetoric” which he said was “directly to blame for the multiple assassination attempts against President Trump”.

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However, Kelly’s depiction of Trump as an undemocratic authoritarian was backed up by Elizabeth Neumann, a former deputy chief of staff in the homeland security department in his administration, who told Politico that he “does not operate by the rule of law”.

“Does he have authoritarian tendencies? Yes,” she said. “Is he kind of leaning towards that ultra-nationalism component? Absolutely.”

Trump’s Republican supporters belittled Kelly’s intervention. Chris Sununu, the Republican governor of New Hampshire, called his portrayal of the former president “an outrageous statement” and said Trump’s own record of extreme statements was “baked in” to the electorate’s assessment.

“I respect John Kelly a lot, but obviously, everyone knows there’s a huge personal relationship divide,” Sununu told NewsNation.

The row overshadowed other developments on the campaign trail, where the Republican nominee extended his catalogue of recent threats to Jack Smith, the special counsel appointed by the justice department to investigate allegations that he tried to overturn the 2020 election and hide classified documents.

Asked by the conservative broadcaster Hugh Hewitt whether he would grant himself a presidential pardon or fire Smith if elected, Trump said: “It’s so easy. I would fire him within two seconds.”

He also noted that “we got immunity from the supreme court,” a reference to a ruling by the court’s conservative majority last June that presidents are immune from prosecution for official acts carried out in the line of duty.