The former president appeared to simulate a “sex act” on his mic after encountering sound issues during a rally at Fiserv Forum in Milwaukee.
Appearing visibly agitated, Trump asked supporters whether they wanted to see him “knock the hell out of people backstage”.
He was handed a new microphone but he complained it was “too low” and performed a gesture that some likened to “fellatio”.
“I get so angry. I’m up here seething. I’m working my ass off with this stupid mic,” he said at one point during the rally.
“I’m blowing out my left arm, now I’m gonna blow out my right arm, and I’m blowing out my damn throat too because of these stupid people.”
Navarro, who has been campaigning for Kamala Harris, said the incident showed Trump was “disgusting, unhinged and unfit to represent the United States”.