When US presidential candidate Donald Trump called a 19-year-old beauty queen names back in 1997, he probably never thought it would come back to haunt him.

But Venezuelan beauty Alicia Machado, who won the Miss Universe Crown in 1996, has got under Trump's skin two decades later.

On early Friday (Sept 30) morning US time, the real estate mogul turned politician posted three tweets attacking Machado, four days after his opponent Hillary Clinton brought her name up in the first presidential debate as an example of Trump's attitude towards women.

"Wow, Crooked Hillary was duped and used by my worst Miss U. Hillary floated her as an 'angel' without checking her past, which is terrible!" he tweeted.

He called Machado, now 39, "disgusting", accused Clinton of helping her become a US citizen, and asked followers to "check out sex tape and past".

Media outlets and Trump supporters have since dug up some of Machado's past association with criminals and risque performances, but no sex tape thus far.

His tweets on Friday were a delayed response to Clinton's charge late in Monday's debate that he called Machado "Miss Piggy" and "Miss Housekeeping".

Trump's response then was, "Where did you find this? Where did you find this?"

The next day, he defended himself on Fox News by saying it was a "real problem" that Machado had gained a "massive amount of weight" after the pageant.

The Republican candidate's tweets were posted in the wee hours of Friday morning, prompting Clinton to tweet: "Who gets up at 3 o'clock in the morning to engage in a Twitter attack against a former Miss Universe?"

"His latest Twitter meltdown is unhinged, even for him," she added.

Machado's response, also on social media, was to double down on how Trump has attacked her now, and when she was young.

She said that his attacks are "cheap lies with bad intentions" and an example of how he "insists on demoralising women, minorities, and people of certain religions through his hateful campaign".

She also said that she became a citizen of the United States, only in August, because her daughter was born there.

Her post was accompanied by a photo of herself draped in an American flag.

Both she and Clinton have pointed out that Trump is not suited for the Oval Office as he is unable to keep his cool when baited.

The feud between Trump and Machado goes way back.

A few months after she won her title in 1996, there was talk that pageant organisers threatened to strip Machado of her crown if she did not lose weight, something that they later denied.

The 1.73m beauty had gone from 51kg to 60kg at one point. Trump claimed she was once about 77kg.

The organisers called her "chubby" and claimed that the extra pounds she put on was an issue for her swimsuit contracts.

Trump, who had just taken over the pageant, famously staged a press conference at a gym in New York where the press was invited to film Machado work out.

He dubbed her an "eating machine", and said: "We've tried diet, spa, a trainer, incentives ... Forget it, the way she's going, she'd eat the whole gymnasium."

Politifact reported that he once called her "the Incredible Bulk".

Machado has said that this fat-shaming caused her to develop eating disorders. It evidently still rankles, although the then-Miss Universe said, on a visit to Singapore in 1997, that she didn't care.

"I don't care about this," she said with a shrug to reporters' questions then. "When people say 'she's fat, not fat, pretty, not pretty', more movie and TV offers come in for me. This is the first time Miss Universe is so famous."

In Singapore to attend the Miss Singapore Universe finals, she had defended her weight, and said that she wanted to show people you could be pretty "with a little more weight".

"Maybe last time I was anorexic. Standing at five feet eight, 49 kg is an unreal weight," she had told The Straits Times.

She even jokingly asked "Where's the chocolate cake?" during the interview, making light of claims that she loved cakes, chocolate and pasta.

After the controversy over her weight, more sinister claims that she was involved in a kidnapping of a baby in Venezuela emerged.

She was accused of helping her then-boyfriend, Juan Rodriguez Reguetti, kidnap his 11-month-old nephew from his brother-in-law, said reports in 1998.

Reguetti's sister, who was the mother of the 11-month-old, committed suicide, and he reportedly held his brother-in-law responsible. He also shot his brother-in-law twice in the head after the funeral service for his sister.

Witnesses said they saw Machado drive the getaway car. She denied the claims in court and was not indicted due to insufficient evidence that she was at the scene.

Later, a judge accused her of threatening to kill him if he went after her boyfriend.

She denied this as well, and said she called the judge to thank him.

The real-life drama did not halt her career in Spanish telenovelas and reality TV.

As for the sex tape, Trump could be referring to a 2005 episode of La Granja, a Spanish reality programme modelled on Big Brother.

She was filmed under the sheets with another contestant, and her then boyfriend, baseball star Bobby Abreu called off their wedding after the salacious clips were shown.

Taking a leaf from Trump's public relations tactics, she has dismissed the bad publicity.

"You know, I have my past. Of course, everybody has a past. I'm not a saint girl. But that is not the point now."

More recently there have been claims that Machado's daughter was fathered by a notorious drug lord, Gerardo Álvarez-Vázquez, who is now behind bars.

She has said her daughter's father is a businessman named Rafael Hernandez Linares.

Machado tweeted on Saturday about Trump again: "Through his attacks, he's attempting to distract from his campaign's real problems and his inability to be the leader of this great country."

Among all the barbs traded at the presidential debate on Monday, Clinton's mention of Machado appears to have struck a nerve.

The episode threatens to damage Trump's already weak standing among women and Hispanics, Reuters said.

A Reuters/Ipsos national tracking poll, released on Friday, showed Clinton leading by 43 per cent to Trump's 38 per cent among likely voters.