Donald Trump has called his former chief of staff John Kelly a “degenerate” and a “lowlife” after the former US Marine Corps general won the support of Kamala Harris for calling his ex-boss a fascist.

Since Kelly's intervention effectively placed the debate over fascism at the center of the US presidential election, the Republican candidate also turned his fire on his Democratic opponent. He falsely accused Harris of calling him Adolf Hitler after the vice president amplified Kelly's comments in a televised speech before endorsing them in a CNN roundtable.

Trump's angry firing came in social media posts amid the fallout from Kelly's comments in an interview with The New York Times, in which he recalled that the former US president had repeatedly praised Hitler's achievements when he was in the White House.

In a separate interview with the Atlantic, Kelly described Trump complaining that he did not have generals who were as loyal as he believed German military commanders were loyal 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 of pure hatred against Trump Derangement Syndrome.”

“This guy had two qualities that didn’t mix well,” he wrote. “He was tough and stupid. John Kelly is a low life.”

Kelly told the Times that Trump “fits the general definition of a fascist” and that he would rule as a dictator if re-elected.

In Wednesday's statement, Harris – who had issued increasingly harsh warnings about Trump's authoritarianism during the campaign amid Trump's increasingly threatening rhetoric – said the interview showed he was seeking “unchecked power.”

She added that it was “deeply disturbing and incredibly dangerous” that he would “invoke” Hitler. She later told CNN anchor 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 about lost.

Trump campaign communications director Steven Cheung accused Harris of “dangerous rhetoric” that he said was “directly responsible for the multiple assassination attempts against President Trump.”

However, Kelly's portrayal of Trump as an undemocratic authoritarian was echoed by Elizabeth Neumann, a former deputy chief of staff at his administration's Department of Homeland Security, who told Politico that he “does not act according to the principles of the rule of law.”

“Does he have authoritarian tendencies? Yes,” she said. “Does he somehow lean towards this ultra-nationalist component? Absolutely.”

Trump's Republican supporters denounced 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 “worked into” voters' assessment.

“I respect John Kelly a lot, but of course everyone knows that there is a big gap in personal relationships,” Sununu told NewsNation.

The dispute overshadowed other developments in the campaign as the Republican candidate expanded his catalog of recent threats against Jack Smith, the special counsel hired by the Justice Department to investigate allegations that he tried to overturn the 2020 election and hide classified documents.

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

He also noted that “we have received immunity from the Supreme Court,” referring to a ruling by the court's conservative majority last June that presidents are protected from prosecution for official acts within the scope of their duties.