Trump's refusal to drop his invective against Alicia Machado, the Miss Universe from Venezuela whom he criticised for gaining weight after she won the crown in 1996, threatened to damage his already weak standing among women and Hispanics.
With less than six weeks to go until the November 8 election, Trump (70) made reference - without giving evidence - to a sex tape involving Machado, who had publicly denounced him all week as a humiliating bully. His attacks provided critics with new fodder to question his temperament for the White House.
"Did Crooked Hillary help disgusting (check out sex tape and past) Alicia M become a US citizen so she could use her in the debate?" Trump said in his Twitter messages in the early hours of Friday. He did not elaborate on his allegations beyond saying that the Clinton campaign was unaware of the beauty queen's past and had been "duped" by her.
At a campaign rally in Coral Springs, Florida, Clinton said the episode was evidence that "a man who can be provoked by a tweet should not have the nuclear codes."
"Who gets up at 3am in the morning to engage in a Twitter attack against a former Miss Universe? I mean he hurled as many insults as he could. Really, why does he do things like that?
"His latest Twitter meltdown is unhinged, even for him. It proves again he is temperamentally unfit to be President of the United States," she said.
Campaign spokesman Nick Merrill said Clinton called Machado on Friday to thank her for the courage she has shown.
Politico reported that Machado has starred in racy reality TV programmes and posed naked for Playboy's Mexican version, but said media outlets have found no evidence that she has starred in pornographic films.
Machado, now a US citizen, dismissed the fresh attacks with a message on Instagram, calling them "cheap lies" from a man intent on defaming her.
"By way of his hate campaign, the Republican candidate insists on discrediting and demoralizing a woman, which is clearly one of his most frightening characteristics," she wrote in her post in Spanish, next to a photo of herself draped in an American flag.
TEMPERAMENT AT PLAY
Clinton raised Trump's treatment of Machado during their first presidential debate on Monday night as an example of how he views women. Clinton said Trump, the former owner of the Miss Universe pageants, had called Machado "Miss Piggy" and also "Miss Housekeeping" because she was a Latina.
The New York real estate mogul attacked Machado the following day in a television interview as he sought to rebound from a debate that Clinton was widely viewed as having won. He also promised to hit Clinton harder in their next debate, on October 9, suggesting he might attack Clinton on the infidelities of her husband, former President Bill Clinton.
Speaking to Fox News on Tuesday, Trump said Machado had gained a "massive amount of weight" after her pageant win and that it was a "real problem."
Critics said his offensive this week raised questions about Trump's character. The former television reality star has never run for office before and his habit of making off-the-cuff insults has alarmed many in the Republican Party establishment.
"When one of the two presidential candidate finds himself in a Twitter war with a former Miss Universe and is attacking her throughout the night ... not only does it demonstrate a lack of restraint but almost an incapacity to control impulses," Republican strategist Steve Schmidt, who is not a Trump supporter, said in a telephone interview on Friday.
In taking the bait Clinton dangled on Monday, Trump echoed his August entanglement in a days-long public dispute with the parents of a Muslim Army captain killed in Iraq. That incident caused anguish among many Republican leaders concerned that he had nothing to gain by attacking a grieving family.
Trump's remarks on Friday could hurt him further with women and Hispanic voters, both groups that favor Clinton in opinion polls. Clinton (68), a former US senator and secretary of state, is the first woman to win the presidential nomination of a major U.S. political party and has led Trump in opinion polls.
On Friday afternoon Trump tweeted a sarcastic reference to the sort of important 3am calls that White House incumbents receive and that were made famous in a 2008 Clinton presidential campaign commercial.
Trump has criticised Clinton as being asleep and missing such a call during the 2012 attack in Benghazi, Libya, when she was secretary of state.
"For those few people knocking me for tweeting at three o'clock in the morning, at least you know I will be there, awake, to answer the call!" Trump said in his tweet.
