Clinton slams Trump's 'unhinged' attack on ex-Miss Universe

She tweeted: "When something gets under Donald's thin skin, he lashes out and can't let go. This is dangerous for a president."

Mr Trump shamed the 1996 Miss Universe Ms Machado winner for her sexual history and encouraged Americans to check out what he called her "sex tape".

A day after the Republican candidate made reference to former president Bill Clinton's infidelities, Mr Trump also accused Hillary Clinton's campaign of helping Ms Machado gain US citizenship.

Mr Trump offered no proof of the claim, but said Ms Machado had a "terrible" past that a "duped" Mrs Clinton had overlooked before holding her up "as an 'angel'" in the first presidential debate.

The Republican wrote on his verified Twitter account: "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?"

Mr Trump's taunt appeared to refer to footage from a Spanish reality show in 2005 in which Ms Machado was a contestant and appeared on camera in bed with a male contestant.

The show took place almost a decade after Mr Trump invited reporters to watch Ms Machado exercise and mocked her publicly for gaining weight after she won Miss Universe, which Mr Trump owned at the time.

In Monday's debate, Democratic candidate Mrs Clinton brought up her rival's taunting of Ms Machado.

With the US election less than 40 days away, Mr Trump's allies have implored him to stick to attacks on Mrs Clinton over her family foundation, her emails controversy or her long history as a political insider.

Rather than backing away from the comments about Ms Machado, Mr Trump has repeatedly pushed the issue back to the centre of the conversation, even as early voting in critical states gets under way.

He said this week that Ms Machado had gained "a massive amount of weight," adding that it was "a real problem".

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

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

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

Clinton spokesman Brian Fallon took to Twitter to ask of Trump: "What kind of human being is this?"

Her campaign chairman, John Podesta, tweeted that as a man of a certain age he shares Mr Trump's urge to get up in the middle of the night, but added a safety tip: "Don't reach for your phone."

A day earlier, Mr Trump had warned voters that a Hillary Clinton victory would bring her husband's sex scandal back to the White House.

"The American people have had it with years and decades of Clinton corruption and scandal. Corruption and scandal," Mr Trump said.

"An impeachment for lying. An impeachment for lying. Remember that? Impeach."

The reference to the Monica Lewinsky scandal of the 1990s came despite Mr Trump's insistence that he has been showing impressive restraint by not bringing it up. He said he declined to mention it during Monday's debate out of respect for the Clintons' daughter, Chelsea, who was in the room.

When she was asked about the possibility that Mr Trump would raise her husband's infidelities, Mrs Clinton said the Republican could run his campaign "however he chooses".

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

Mrs Clinton has delivered a mostly positive message in the days since her debate performance re-energised her candidacy.

She is campaigning in Florida, a key battleground, where she us planning a speech focused on national service in Fort Pierce and another event in Coral Springs.