Mr. Kelly, Trump’s longest-serving chief of staff, said the then-president told him that “Hitler did some good things,” the New York Times reported.

Mrs. Clinton’s comments drew outrage from the Trump campaign, which called the comparison between Trump supporters and Nazis “disgusting.”

“Hillary Clinton is so shaken by her eight-year case of Anti-Trump Confusion Syndrome that she has forgotten that SHE hosted an event at Madison Square Garden as a senator and that her husband Bill accepted the Democratic nomination there.” Karoline Leavitt, the Trump campaign’s national press secretary, told the New York Post.

“Aside from her hypocrisy, Hillary’s rhetoric is abhorrent to half the country,” she added.

Mrs. Clinton’s comments echoed those of Kamala Harris, who said at a town hall event this week that she believed Trump was “a fascist.”

Republicans condemned the vice president’s comments, accusing her of “stoking the fire under a boiling cauldron of political hostility.”

Republicans Mike Johnson, the House speaker, and Mitch McConnell, the Senate minority leader, issued a joint statement saying their rhetoric “risks inviting another potential assassin” to target Trump , and American lives “at risk.”

“We call on the Vice President to take these threats seriously, stop the escalation of the threat environment, and help ensure that President Trump has the necessary resources to protect himself from these threats,” they said.

Mrs. Clinton, who is not yet on the Democratic ticket, is currently promoting her new book, “Something Lost, Something Gained,” which came out last month.

The highest-profile appearance she has made on behalf of Ms. Harris since endorsing her was a prime-time speech at the Democratic National Convention in August, when she smiled and nodded as the crowd chanted about Trump: “Lock him up!”

Mrs. Clinton’s comments are being compared to those she made in the late stages of her failed 2016 presidential campaign, when she said half of Trump’s supporters belonged in a “basket full of deplorables.”

The outcry over these comments was widely seen as a turning point in the election and one of the reasons for her electoral defeat.

According to NBC, Clinton is expected to join the Harris campaign in the final week before the Nov. 5 election.