"She took a short-circuit in the brain. She's got problems," Trump said, seizing on Clinton's explanation that she "short-circuited" a recent answer about her truthfulness in discussing her email server.
"Honestly, I don't think she's all there," he added.
The attacks flowed from the Republican nominee as he once again tore into Clinton as "unstable," "unbalanced" and "totally unhinged."
Trump's stepped-up attacks on Clinton come as he has been falling in a slew of recent battleground states
and national polls
and as top Republicans have fretted
about Trump repeatedly knocking himself off message by engaging in controversies rather than focusing on Clinton.
While Trump in the last week escalated a feud with the parents of a fallen U.S. soldier and opened a party rift by saying he was not yet ready to endorse the Republican speaker of the House, Trump on Friday launched into a lengthy and focused attack on Clinton during his rallies, and built on those attacks on Saturday.
"She's a liar. She is a horrible, horrible human being," Trump told a crowd of supporters gathered in a sweltering New Hampshire high school gym. "She's incompetent and I don't' think that you can even think of allowing this woman to become president of the United States."
But before taking the stage in New Hampshire, Trump previewed the "short-circuit" line of attack online, tweeting earlier Saturday that "anybody whose mind 'SHORT CIRCUITS' is not fit to be our president! Look up the word 'BRAINWASHED.'"
And in a video posted on his Facebook page earlier Saturday, Trump's campaign suggested Clinton was "melting down," calling her "robot Hillary."
Clinton's use of the term "short-circuited" came as she answered a question Friday at a gathering of black and Hispanic journalists about her recent assertion in a Fox News interview that FBI Director James Comey said she had been "truthful" in discussing her use of a private email server during her time as secretary of state. Clinton's claim in that interview has been widely debunked as false.
"I was pointing out in both of those instances, that Director Comey had said that my answers in my FBI interview were truthful. That really is the bottom line here," she said. "What I told the FBI, which he said was truthful, is consistent with what I have said publicly. I may have short-circuited, and for that, I will try to clarify."
Stumping Saturday night, Trump also alleged that the terrorist group ISIS is dreaming of a Clinton presidency.
"Remember, remember, remember ISIS is looking, folks. They dream of Hillary Clinton," Trump said. "They look at her and they say this can't be happening to us. How great is this."
Trump's latest barrage of attacks against Clinton -- while not a departure from his brand of personal and aggressive attacks against his opponents -- did mark an escalation in his attacks, just as Clinton and her allies are stepping up their attacks against Trump.
Clinton has accused Trump of being "temperamentally unfit" to be president and a slew of top former government officials have raised questions about Trump's character and his fitness to become the next commander-in-chief.
Michael Morell, the former head of the CIA, wrote in a New York Times op-ed
that Trump "is not only unqualified for the job, but he may well pose a threat to our national security."
"The character traits he has exhibited during the primary season suggest he would be a poor, even dangerous, commander in chief," Morell wrote, pointing to Trump's "obvious need for self-aggrandizement, his overreaction to perceived slights" and "his routine carelessness with the facts."
Trump also pushed back on those attacks on Saturday.
"I get a kick out of these dopey, dopey, dopey people. These stupid foolish people when they talk about, "Can we trust Donald Trump with nuclear? Can we trust him?" Trump said. "You know, it's a whole little narrative. They'll spend a billion dollars on this but the people aren't buying it."