US Democratic candidate Joe Biden has flatly denied sexually assaulting a former staff assistant, Tara Reade, nearly 30 years ago.

"I'm saying unequivocally: it never, never happened," he said of the allegations during a TV interview on Friday.

He asked for a search of the Senate archives for any record of a complaint Ms Reade allegedly filed at the time.

Ms Reade made a criminal complaint to police last month.

She said she was a victim of sexual assault without naming Mr Biden. The police complaint, she said, was filed "for safety reasons only" as the statute of limitations for her claim had expired.

Mr Biden is running against Republican President Donald Trump, who has been accused of sexual misconduct by some 25 women.

The Democrat put out his statement before speaking on Morning Joe, a show on US cable channel MSNBC.

What are Reade's accusations?

She was working as a staff assistant to Mr Biden from 1992-93, when he was a senator for the US state of Delaware.

Ms Reade, now 56, says that in 1993 he forced her up against a wall in the halls of Congress, and put his hands under her shirt and skirt, penetrating her with a finger.

"I remember him saying, first, as he was doing it 'Do you want to go somewhere else?' and then him saying to me, when I pulled away... he said 'Come on man, I heard you liked me,'" she told podcast host Katie Halper in March. "That phrase stayed with me."

Ms Reade says records of Mr Biden's 36-year career as a US senator will contain evidence that she complained to her superiors about him.

The records are being held at the University of Delaware, which has said it will not release any papers until two years after Mr Biden leaves public life.

It appears there were no witnesses to the alleged assault but three people have backed Ms Reade's account.

Her brother, a former neighbour and a former colleague have all said that they heard her give it shortly after the alleged incident.

Former neighbour Lynda LaCasse told Business Insider: "This happened, and I know it did because I remember talking about it."

"I remember her saying, here was this person that she was working for and she idolised him," Ms LaCasse said. "I remember the skirt. I remember the fingers. I remember she was devastated."

Ms Reade is one of more than half a dozen women who have forward over the last year to accuse him of inappropriate touching, hugging or kissing, though none described his actions as sexual assault at the time.

What did Biden say?

Speaking to MSNBC's Mika Brezezinski on Friday, the former vice-president denied any sexual misconduct against Tara Reade outright.

"It did not happen. Period," he said.

Brezezinski pressed Mr Biden on his former statements suggesting that women should be believed when coming forward with their stories of sexual violence.

In 2018, when now-Supreme Court Justice Brett Kavanaugh was accused of sexual assault by Christine Blasey Ford, Mr Biden was joined by a chorus of top Democrats in supporting Dr Ford's claims, and insisting that she be heard.

"For a woman to come forward in the glaring light of focus, nationally, you've got to start off with the presumption that at least the essence of what she's talking about is real, whether or not she forgets the facts, whether or not it's been made worse or better over time," Mr Biden told reporters at the time.

"Are women to be believed unless it pertains to you?" Brezezinski asked Mr Biden on Friday.

"Women are to be believed, given the benefit of the doubt," Mr Biden said. "Then you have to look at the circumstances and the facts."

"The truth matters."

Mr Biden refused to speculate on Ms Reade's motives, saying she had a right to come forward "and say whatever she wants to say. But I have a right to say: 'Look at the facts.'"

But the former vice-president said that his files housed at the University of Delaware contained no personnel information that would pertain to Ms Reade. Despite repeated questioning from Brezezinski, Mr Biden insisted those 1,800 boxes of documents did not contain any personnel files and would be used as political "fodder" for his ongoing presidential campaign.

Shortly after his MSNBC interview, Mr Biden wrote a letter to the secretary of the US Senate asking the office to check its files for any record of Ms Reade's complaint.

Mr Biden asks in the letter for "assistance in determining whether 27 years ago a staff member in my United States Senate office filed a complaint alleging sexual harassment" and whether it is located in the US National Archives.

The letter asks that any unearthed documents be made public and that "the public release include not only a complaint if one exists, but any and all other documents in the records that relate to the allegation".

Standing by Their Man

Analysis by Tara McKelvey, BBC White House reporter

As Joe Biden faces a serious sexual assault accusation, Democrats face a problem.

They are trying to be the political party that champions women and survivors of sexual assault, and yet their 2020 presidential candidate-apparent himself has been caught up in controversy over how he treats women.

Though Mr Biden has insisted that the allegation that he assaulted his former staff assistant Tara Reade 27 years ago is "unequivocally" false, he - and his Democratic supporters - are finding it hard to avoid the accusation that there is a whiff of hypocrisy over the whole affair.

Conservatives have pointed out the relative silence over Ms Reade's allegation compared to the outcry over sexual assault claims made by Christine Blasey Ford, who accused Brett Kavanaugh, President Trump's most recent Supreme Court appointment.

In September 2018, while the Kavanaugh hearing was unfolding, Ford's name came up 1,898 times on CNN and nearly as often on MSNBC, according to RealClearPolitics.

Until recently, however, Ms Reade's name was barely mentioned.

Mr Biden himself said Ms Ford and other women who come forward with sexual assault claims should be presumed to be telling the truth. He is having trouble answering questions now over why it should not be the case with Ms Reade.

That has posed a particular problem for senior Democratic women backing Mr Biden - some of whom are being whispered as possible names for the Biden 2020 ticket.

The senators Kamala Harris and Elizabeth Warren, former Georgia gubernatorial candidate Stacey Abrams are said to be among those hoping to become his vice-presidential candidate.

But with the sexual misconduct claims, an offer from Biden will be a "poisoned chalice", says Rebecca Traister in New York Magazine: Once one of these women accepts his offer and runs as his vice-presidential candidate, they will be tied to Biden's "historical shortcomings" and could imperil themselves and their own political future. It is a fine mess for progressive women to fall into, with no easy way out.

Could the allegations hurt Biden's election chances?

Some Republicans are seizing on the Reade allegations to portray the Democrats as hypocrites who only defend women who allege wrongdoing against conservatives, the Associated Press reports.

President Trump's campaign derided Mr Biden's interview. The Democrat "once again demonstrated that he believes he should be held to a different standard than he has set for others", a Trump campaign spokeswoman said.

But given the longstanding allegations against Donald Trump, a man who once boasted of grabbing women by the genitals, the Republican camp may struggle to make political capital from Mr Biden's troubles.

At the same time, the Democrats have set themselves up as the party of moral purity, on gender as well as race. The party's politicians are inevitably held to a different standard.

And women are a core constituency for the party, traditionally giving more votes to Democratic candidates than Republicans.