"The people of this country don't want somebody who's going to short circuit up here," Trump told supporters in New Hampshire.
"She's a dangerous liar," the real estate mogul said of the Democratic former secretary of state. "She is a totally unhinged person. She's unbalanced."
Trump's supporters chanted "Lock her up!" during the event in the town of Windham, which was broadcast online.
On Friday, Clinton acknowledged she "may have short-circuited" when she tried to clarify statements about her use of a private email server.
The Democrat had told Fox News Sunday that FBI director James Comey said her "answers were truthful" about whether she sent or received classified material via private email. That comment was branded false by Washington-based fact-checkers.
Trump, 70, has embraced her phrasing as a line of attack. On Saturday ahead of his rally in New Hampshire, he posted a campaign ad on his Facebook page that refers to Clinton as "Robot Hillary" -- and shows sparks flying out of her mouth.
"Is Robot Hillary melting down?" the ad asks.
Trump is trying to boost his campaign after a week of missteps that saw his poll numbers drop and the 68-year-old Clinton, a former senator and first lady, seize the momentum.
He reversed course and endorsed House Speaker Paul Ryan for re-election on Friday, pledging to work with Republican Party leaders. Trump also said he would support veteran Republican Senator John McCain, with whom he has sparred in the past.
"We need unity. We have to win this election," Trump told a rally in Ryan's state of Wisconsin.
