Two women have told the New York Times that Donald Trump touched them inappropriately, but the Trump campaign has dismissed the report.
One woman said Mr Trump grabbed her breasts and attempted to put his hand up her skirt on a flight three decades ago.
The second woman said Mr Trump kissed her on the mouth against her will at Trump Towers in 2005.
Mr Trump's campaign said "this entire article is fiction".
The New York Times was launching "a completely false, coordinated character assassination," a statement from his campaign said.
Jessica Leeds, 74, from Manhattan, told the paper she had been sitting next to Mr Trump in the first-class cabin of a flight to New York when he lifted the armrest and began to touch her.
"He was like an octopus... his hands were everywhere," she said. "It was an assault."
Rachel Crooks was a 22-year-old receptionist at a real estate company in Trump Tower when she said she was kissed on the lips by Mr Trump outside a lift in the building.
"It was so inappropriate," Ms Crooks told the New York Times.
"I was so upset that he thought I was so insignificant that he could do that."
Neither women had ever reported their accounts to the authorities, but both shared what happened to them with friends and family.
Ms Leeds said she was "angry and shook up" after Mr Trump touched her.
She said she quickly left the first-class cabin and returned to economy.
Ms Leeds said she had largely put the encounter out of her mind until last year, when Mr Trump's presidential campaign took off.
"His behaviour is deep-seated in his character," she said. "To those who would vote for him, I would wish for them to reflect on this."
Ms Crooks said she "started hysterically crying" to her boyfriend after Mr Trump kissed her.
She said Mr Trump later asked for her phone number, which he said he wanted to pass on to his modelling agency, but she never heard from them.
On coming forward, she said: "People should know... this behaviour is pervasive and it is real."
The Clinton campaign said the "disturbing" New York Times story "sadly fits everything we know about the way Donald Trump has treated women".
"These reports suggest that he lied on the debate stage and that the disgusting behaviour he bragged about in the tape is more than just words."
Meanwhile, Mindy McGillivray told the Palm Beach Post she was 23 when Trump grabbed her bottom at his Mar-a-Lago club in Florida in 2003.
She said he was standing next to his then-fiancee Melania Knauss at the time.
Ms McGillivray, who is now 36, said she never reported the incident to the authorities.
But her companion that day, photographer Ken Davidoff, told the newspaper he remembered her pulling him aside moments afterwards and telling him: "Donald just grabbed my ass!"
Mr Trump's campaign said Ms McGillivray's allegation "lacks any merit or veracity".
All three women have said they support Hillary Clinton's campaign for president.
He apologised for the comments, but described them as "locker-room talk".
Many top Republicans, including House Speaker Paul Ryan, have distanced themselves in recent days from the businessman-turned-politician.
Mr Trump fired back in a string of tweets, saying the "shackles" had been removed.
He said he neither wanted nor needed Mr Ryan's support and vowed to teach "disloyal" Republicans "how to win".
A recent PRRI/Atlantic poll suggested Mrs Clinton held a 49-38 lead over her opponent.