Donald Trump has attacked a former beauty pageant winner who criticised him for alleged sexist and misogynistic remarks as "disgusting".
In a stream of tweets, the Republican presidential nominee urged Americans to examine Alicia Machado's personal history and her "sex tape".
He implied his Democratic rival Hillary Clinton had secured US citizenship for Venezuelan-born Ms Machado.
In response, Mrs Clinton tweeted: "This is unhinged, even for Trump."
Ms Machado says she was called "Miss Piggy" by the Republican candidate for the White House when he owned the Miss Universe beauty pageant after she put on weight after winning the title in 1996. She also says he called her "Miss Housekeeping" because of her Latina heritage.
Her case was raised by Mrs Clinton in the first presidential debate earlier this week as an example of Mr Trump's attitude to women.
In his early-morning Twitter outburst, Mr Trump said Ms Machado had a "terrible" past that a "duped" Mrs Clinton had overlooked before holding her up "as an 'angel'".
He added: "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?"
In her Twitter response, Mrs Clinton asked: "What kind of man stays up all night to smear a woman with lies and conspiracy theories?"