Donald Trump accuses beauty queen and Miss Universe Alicia Machado of sex tape

Donald Trump has attempted to shame a former beauty pageant winner for her sexual history and encouraged Americans to check out what he called her "sex tape," in an early-morning tweet-storm that dragged him further away from his campaign's efforts to broaden its appeal to women.

A day after he injected former US President Bill Clinton's dalliances into the campaign, Trump accused Hillary Clinton's campaign of helping 1996 Miss Universe winner Alicia Machado get US citizenship, but offered no proof.

He said Machado had a "terrible" past that a "duped" Mrs Clinton had overlooked before holding her up "as an angel" in the first presidential debate.

"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?" read a missive from Trump posted on his verified Twitter account at 5:30am (7:30pm AEST).

Trump's taunt appeared to refer to footage from a Spanish reality show in 2005 in which Machado was a contestant and appeared on camera in bed with a male contestant. The images, posted this week to a newspaper's website, are grainy and do not include nudity.

The show took place almost a decade after Trump invited reporters to watch Machado exercise and mocked her publicly for gaining weight after she won Miss Universe, which Trump owned at the time. In Monday's debate, Mrs Clinton brought up Trump's taunts of Machado.

After the Twitter tirade against Machado and Clinton, Trump offered a bizarre defence for the rant.

"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!," he Tweeted, suggesting that his unusual waking hours made him a better candidate for the presidency.

Shaming Machado over intimate details from her past was particularly risky as Trump tries to win over more female voters, many of whom are turned off by such personal attacks. It also risks calling further attention to the thrice-married Trump's own history with women.

The outburst was an extreme reminder of how Trump has seemed unable to restrain himself from veering into unhelpful territory, even with the election less than 40 days away. Trump's allies have implored him to stick to attacks on Mrs Clinton over her family foundation, her emails or her long history as a political insider, critiques that fall further out of view whenever Trump sparks a new controversy.

Rather than back way from the comments about Machado, Trump has repeatedly pushed it back to the centre of the conversation, even as early voting in critical states gets underway. He said this week that Machado had gained "a massive amount of weight," adding that it was "a real problem."

On Friday, Trump said Mrs Clinton had been "set up by a con" in holding up Machado "as an 'angel' without checking her past, which is terrible!"

He suggested Mrs Clinton had helped the Venezuela-born actress gain citizenship.

Mrs Clinton's campaign, which has released videos featuring Machado and arranged for reporters to interview her, has highlighted her status as a new American and her plans to cast her first vote for Mrs Clinton.

Mrs Clinton's spokesman Brian Fallon took to Twitter to ask of Trump, "What kind of human being is this?" And her campaign chairman, John Podesta, tweeted that as a man of a certain age he shares Trump's urge to get up in the middle of the night but added a "safety tip" - "don't reach for your phone."

The flurry on Twitter began shortly after 3am (5pm AEDT) on the East Coast when Trump complained about stories regarding his campaign that were based on anonymous sources and told his supporters not to believe them.

"There are no sources, they are just made up lies!" he wrote.

Trump has repeatedly gotten himself in trouble with his late-night and early-morning tweets, which appear to be written by the candidate himself. The reality TV show star once said that aides help him tweet during the day but that in the evenings, he tweets by himself.

Trump has, at various points during his campaign, toned down the content of his tweets, but rarely for long.

His latest broadside against Machado adds fuel to a burgeoning debate in America about "slut-shaming," or putting down women over the perception of promiscuity. Women's advocates have said the phenomenon, which takes place largely online, holds women to a different standard because men are often praised for having multiple partners.

A day earlier, Trump had warned voters that a Hillary Clinton victory would bring her husband's sex scandal back to the White House. It was Trump's latest effort to bounce back from Monday's debate performance, which was widely panned for being less effective than Mrs Clinton's.

"The American people have had it with years and decades of Clinton corruption and scandal. Corruption and scandal," Trump said on Thursday. "An impeachment for lying. An impeachment for lying. Remember that? Impeach."

The fresh rehash of the 1990s Monica Lewisnky scandal came despite Trump's insistence that he's been showing impressive restraint by not bringing it up. Trump has said he declined to mention it during the debate out of respect for Mrs Clinton's daughter, Chelsea, who was in the room.

Mrs Clinton has tried largely to stay out of the fracas over Trump's comments about women and her husband, hoping not to get in her opponent's way while he keeps stepping into controversies her campaign sees as damaging his prospects. Asked on Thursday about the possibility that Trump would raise her husband's infidelities, Mrs Clinton said he could run his campaign "however he chooses."

"That's up to him. I'm going to keep talking about the stakes in this election," Mrs Clinton said.

Trying to draw a contrast with Trump, Mrs Clinton has delivered a mostly positive message in the days since her debate performance re-energised her candidacy.

- additional reporting from AP